"Honey, I Shrunk Docker!" - part 1/3

Apr 7, 2020 • 14 min

This is the first part of our serie of blogposts dealing with Docker image size optimization:


When getting started with containers, it’s pretty easy to be shocked by the size of the images that we build. We’re going to review a number of techniques to reduce image size, without sacrificing developers’ and ops’ convenience. In this first part, we will talk about multi-stage builds, because that’s where anyone should start if they want to reduce the size of their images. We will also explain the differences between static and dynamic linking, as well as why we should care about that. This will be the occasion to introduce Alpine.

In the second part, we will see some particularities relevant to various popular languages. We will talk about Go, but also Java, Node, Python, Ruby, and Rust. We will also talk more about Alpine and how to leverage it across the board.

In the third part, we will cover some patterns (and anti-patterns!) relevant to most languages and frameworks, like using common base images, stripping binaries and reducing asset size. We will wrap up with some more exotic or advanced methods like Bazel, Distroless, DockerSlim, or UPX. We will see how some of these will be counter-productive in some scenarios, but might be useful in some particular cases.

Note that the sample code, and all the Dockerfiles mentioned here, are conveniently available in a public GitHub repository, with a Compose file to build all the images and easily compare their sizes.


How to reduce docker image size?

I bet that everyone who built their first Docker image that compiled some code was surprised (not in a good way) by the size of that image.

Look at this trivial “hello world” program in C:

/* hello.c */
int main () {
puts("Hello, world!");
return 0;

We could build it with the following Dockerfile:

FROM gcc
COPY hello.c .
RUN gcc -o hello hello.c
CMD ["./hello"]

… But the resulting image will be more than 1 GB, because it will have the whole gcc image in it!

If we use e.g. the Ubuntu image, install a C compiler, and build the program, we get a 300 MB image; which looks better, but is still way too much for a binary that, by itself, is less than 20 kB:

$ ls -l hello
-rwxr-xr-x 1 root root 16384 Nov 18 14:36 hello

Same story with the equivalent Go program:

package main

import "fmt"

func main () {
fmt.Println("Hello, world!")

Building this code with the golang image, the resulting image is 800 MB, even though the hello program is only 2 MB:

$ ls -l hello
-rwxr-xr-x 1 root root 2008801 Jan 15 16:41 hello

There has to be a better way!

Let’s see how to drastically reduce the size of these images. In some cases, we will achieve 99.8% size reduction (but we will see that it’s not always a good idea to go that far).

Pro Tip: To easily compare the size of our images, we are going to use the same image name, but different tags. For instance, our images will be hello:gcc, hello:ubuntu, hello:thisweirdtrick, etc. That way, we can run docker images hello and it will list all the tags for that hello image, with their sizes, without being encumbered with the bazillions of other images that we have on our Docker engine.

Multi-stage Docker builds

This is the first step (and the most drastic) to reduce the size of our images. We need to be careful, though, because if it’s done incorrectly, it can result in images that are harder to operate (or could even be completely broken).

Multi-stage builds come from a simple idea: “I don’t need to include the C or Go compiler and the whole build toolchain in my final application image. I just want to ship the binary!”

We obtain a multi-stage build by adding another FROM line in our Dockerfile. Look at the example below:

FROM gcc AS mybuildstage
COPY hello.c .
RUN gcc -o hello hello.c
FROM ubuntu
COPY --from=mybuildstage hello .
CMD ["./hello"]

We use the gcc image to build our hello.c program. Then, we start a new stage (that we call the “run stage”) using the ubuntu image. We copy the hello binary from the previous stage. The final image is 64 MB instead of 1.1 GB, so that’s about 95% size reduction:

$ docker images minimage
minimage hello-c.gcc ... 1.14Go
minimage hello-c.gcc.ubuntu ... 64.2Mo

Not bad, right? We can do even better. But first, a few tips and warnings.

You don’t have to use the AS keyword when declaring your build stage. When copying files from a previous stage, you can simply indicate the number of that build stage (starting at zero).

In other words, the two lines below are identical:

COPY --from=mybuildstage hello .
COPY --from=0 hello .

Personally, I think it’s fine to use numbers for build stages in short Dockerfiles (say, 10 lines or less), but as soon as your Dockerfile gets longer (and possibly more complex, with multiple build stages), it’s a good idea to name the stages explicitly. It will help maintenance for your team mates (and also for future you who will review that Dockerfile months later).

Warning: use classic images

I strongly recommend that you stick to classic images for your “run” stage. By “classic”, I mean something like CentOS, Debian, Fedora, Ubuntu; something familiar. You might have heard about Alpine and be tempted to use it. Do not! At least, not yet. We will talk about Alpine later, and we will explain why we need to be careful with it.

Warning: COPY --from uses absolute paths

When copying files from a previous stage, paths are interpreted as relative to the root of the previous stage.

The problem appears as soon as we use a builder image with a WORKDIR, for instance the golang image.

If we try to build this Dockerfile:

FROM golang
COPY hello.go .
RUN go build hello.go
FROM ubuntu
COPY --from=0 hello .
CMD ["./hello"]

We get an error similar to the following one:

COPY failed: stat /var/lib/docker/overlay2/1be...868/merged/hello: no such file or directory

This is because the COPY command tries to copy /hello, but since the WORKDIR in golang is /go, the program path is really /go/hello.

If we are using official (or very stable) images in our build, it’s probably fine to specify the full absolute path and forget about it.

However, if our build or run images might change in the future, I suggest to specify a WORKDIR in the build image. This will make sure that the files are where we expect them, even if the base image that we use for our build stage changes later.

Following this principle, the Dockerfile to build our Go program will look like this:

FROM golang
COPY hello.go .
RUN go build hello.go
FROM ubuntu
COPY --from=0 /src/hello .
CMD ["./hello"]

If you’re wondering about the efficiency of multi-stage builds for Golang, well, they let us go (no pun intended) from a 800 MB image down to a 66 MB one:

$ docker images minimage
minimage hello-go.golang ... 805Mo
minimage hello-go.golang.ubuntu-workdir ... 66.2Mo

Using FROM scratch

Back to our “Hello World” program. The C version is 16 kB, the Go version is 2 MB. Can we get an image of that size?

Can we build an image with just our binary and nothing else?

Yes! All we have to do is use a multi-stage build, and pick scratch as our run image. scratch is a virtual image. You can’t pull it or run it, because it’s completely empty. This is why if a Dockerfile starts with FROM scratch, it means that we’re building from scratch, without using any pre-existing ingredient.

This gives us the following Dockerfile:

FROM golang
COPY hello.go .
RUN go build hello.go

FROM scratch
COPY --from=0 /go/hello .
CMD ["./hello"]

If we build that image, its size is exactly the size of the binary (2 MB), and it works!

There are, however, a few things to keep in mind when using scratch as a base.

No shell

The scratch image doesn’t have a shell. This means that we cannot use the string syntax with CMD (or RUN, for that matter).

Consider the following Dockerfile:

FROM scratch
COPY --from=0 /go/hello .
CMD ./hello

If we try to docker run the resulting image, we get the following error message:

docker: Error response from daemon: OCI runtime create failed:
container_linux.go:345: starting container process caused
"exec:\"/bin/sh\": stat /bin/sh: no such file or directory": unknown.

It’s not presented in a very clear way, but the core information is here: /bin/sh is missing from the image.

This happens because when we use the string syntax with CMD or RUN, the argument gets passed to /bin/sh. This means that our CMD ./hello above will execute /bin/sh -c./hello”, and since we don’t have /bin/sh in the scratch image, this fails.

The workaround is simple: use the JSON syntax in the Dockerfile. CMD ./hello becomes CMD ["./hello"]. When Docker detects the JSON syntax, it runs the arguments directly, without a shell.

No debugging tools

The scratch image is, by definition, empty; so it doesn’t have anything to help us troubleshoot the container. No shell (as we said in the previous paragraph) but also no ls, ps, ping, and so on and so forth. This means that we won’t be able to enter the container (with docker exec or kubectl exec) to look into it.

(Note that strictly speaking, there are some methods to troubleshoot our container anyway. We can use docker cp to get files out of the container; we can use docker run --net container: to interact with the network stack; and a low-level tool like nsenter can be very powerful. Recent versions of Kubernetes have the concept of ephemeral container, but it’s still in alpha. And let’s keep in mind that all these techniques will definitely make our lives more complicated, especially when we have so much to deal with already!)

One workaround here is to use an image like busybox or alpine instead of scratch. Granted, they’re bigger (respectively 1.2 MB and 5.5 MB), but in the grand scheme of things, it’s a small price to pay if we compare it to the hundreds of megabytes, or the gigabytes, of our original image.

No libc

That one is trickier to troubleshoot. Our simple “hello world” in Go worked fine, but if we try to put a C program in the scratch image, or a more complex Go program (for instance, anything using network pacakges), we will get the following error message:

standard_init_linux.go:211: exec user process caused "no such file or directory"

Some file seems to be missing. But it doesn’t tell us which file is missing exactly.

The missing file is a dynamic library that is necessary for our program to run.

What’s a dynamic library and why do we need it?

After a program is compiled, it gets linked with the libraries that it is using. (As simple as it is, our “hello world” program is still using libraries; that’s where the puts function comes from.) A long time ago (before the 90s), we used mostly static linking, meaning that all the libraries used by a program would be included in the binary. This is perfect when software is executed from a floppy disk or a cartridge, or when there is simply no standard library. However, on a timesharing system like Linux, we run many concurrent programs that are stored on a hard disk; and these programs almost always use the standard C library.

In that scenario, it gets more advantageous to use dynamic linking. With dynamic linking, the final binary doesn’t contain the code of all the libraries that it uses. Instead, it contains references to these libraries, like “this program needs functions cos and sin and tan from libtrigonometry.so. When the program is executed, the system looks for that libtrigonometry.so and loads it alongside the program so that the program can call these functions.

Dynamic linking has multiple advantages.

  1. It saves disk space, since common libraries don’t have to be duplicated anymore.
  2. It saves memory, since these libraries can be loaded once from disk, and then shared between multiple programs using them.
  3. It makes maintenance easier, because when a library is updated, we don’t need to recompile all the programs using that library.

(If we want to be thorough, memory savings aren’t a result of dynamic libraries but rather of shared libraries. That being said, the two generally go together. Did you know than on Linux, dynamic library files typically have the extension .so, which stands for shared object? On Windows, it’s .DLL, which stands for Dynamic-link library.)

Back to our story: by default, C programs are dynamically linked. This is also the case for Go programs that are using some packages. Our specific program uses the standard C library, which on recent Linux systems will be in file libc.so.6. So to run, our program needs that file to be present in the container image. And if we’re using scratch, that file is obviously absent. This is the same if we use busybox or alpine, because busybox doesn’t contain a standard library, and alpine is using another one, that is incompatible. We’ll tell more about that later.

How do we solve this? There are at least 3 options.

Building a static binary

We can tell our toolchain to make a static binary. There are various ways to achieve that (depending on how we build our program in the first place), but if we’re using gcc, all we have to do is add -static to the command line:

gcc -o hello hello.c -static

The resulting binary is now 760 kB (on my system) instead of 16 kB. Of course, we’re embedding the library in the binary, so it’s much bigger. But that binary will now run correctly in the scratch image.

We can get an even smaller image if we build a static binary with Alpine. The result is less than 100 kB!

Adding the libraries to our image

We can find out which libraries our program needs with the ldd tool:

$ ldd hello
linux-vdso.so.1 (0x00007ffdf8acb000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007ff897ef6000)
/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007ff8980f7000)

We can see the libraries needed by the program, and the exact path where each of them was found by the system.

In the example above, the only “real” library is libc.so.6. linux-vdso.so.1 is related to a mechanism called VDSO (virtual dynamic shared object), which accelerates some system calls. Let’s pretend it’s not there. As for ld-linux-x86-64.so.2, it’s actually the dynamic linker itself. (Technically, our hello binary contains information saying, “hey, this is a dynamic program, and the thing that knows how to put all its parts together is ld-linux-x86-64.so.2”.)

If we were so inclined, we could manually add all the files listed above by ldd to our image. It would be fairly tedious, and difficult to maintain, especially for programs will lots of dependencies. For our little hello world program this would work fine. But for a more complex program, for instance something using DNS, we would run into another issue. The GNU C library (used on most Linux systems) implements DNS (and a few other things) through a fairly complex mechanism called the Name Service Switch (NSS in short). This mechanism needs a configuration file, /etc/nsswitch.conf, and additional libraries. But these libraries don’t show up with ldd, because they are loaded later, when the program is running. If we want DNS resolution to work correctly, we still need to include them! (These libraries are typically found at /lib64/libnss_*.)

I personally can’t recommend going that route, because it is quite arcane, difficult to maintain, and it might easily break in the future.

Using busybox:glibc

There is an image designed specifically to solve all these issues: busybox:glibc. It is a small image (5 MB) using busybox (so providing a lot of useful tools for troubleshooting and operations) and providing the GNU C library (or glibc). That image contains precisely all these pesky files that we were mentioning earlier. This is what we should use if we want to run a dynamic binary in a small image.

Keep in mind, however, that if our program uses additional libraries, these libraries will need to be copied as well.

Recap and (partial) conclusion

Let’s see how we did for our “hello world” program in C. Spoiler alert: this list includes results obtained by leveraging Alpine in ways that will be described in the next part of this series.

  • Original image built with gcc: 1.14 GB
  • Multi-stage build with gcc and ubuntu: 64.2 MB
  • Static glibc binary in alpine: 6.5 MB
  • Dynamic binary in alpine: 5.6 MB
  • Static binary in scratch: 940 kB
  • Static musl binary in scratch: 94 kB That’s a 12000x size reduction, or 99.99% less disk space.

Not bad.

Personally, I wouldn’t go with the scratch images (because troubleshooting them might be, well, trouble) but if that’s what you’re after, they’re here for you!

In the next part, we will mention some aspects specific to the Go language, including cgo and tags. We will also cover other popular languages, and we will talk more about Alpine, because it’s pretty awesome if you ask me.

Next part: “Honey, I Shrunk Docker!”, Part 2/3

This article was originally published here

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