Building a Docker image often involves installing packages or downloading code, and if you’re installing private code you often need to gain access with a secret: a password, a private key, a token. You don’t want those secrets to end up in the final image, though; if it’s in the image, anyone with access to the image can extract it.

Just to be clear, the goal here is to get build secrets in. Runtime secrets have existing and sufficient mechanisms already (environment variables and volumes).

In this article I’ll cover:

The eventual, future solution. Some broken or problematic solutions you can use now, but probably shouldn’t. The sneaky solution: getting secrets in through the network.

Note: Outside the topic under discussion, the Dockerfiles in this article are not examples of best practices, since the added complexity would obscure the main point of the article. To ensure you’re writing secure, correct, fast Dockerfiles, consider my quickstart guide, which includes 60+ best practices, from security to performance.

The future solution: BuildKit

Docker is currently working on a new build system called BuildKit, which includes support for adding secrets, as well as for SSH agent authentication forwarding.

BuildKit doesn’t seem ready for real-world use, however. It’s off by default in the 18.09 release (from September 2018), the May 2019 update is still fixing user-impacting bugs, and the bug tracker for Docker still has lots and lots of BuildKit issues.

At some point it will be ready and on by default, but we’re not there yet.

Existing solutions you don’t want to use

Unavailable options: environment variables and volumes

If you’re wondering why not just use environment variables, or volumes, it’s because the goal is to use these secrets as part of docker build , e.g. to download a package that needs to be installed as part of the build. And unlike docker run , docker build doesn’t support environment variables or volumes.

COPY the secret in as a file

If you do this, the secret ends up in one image’s layers, even if you delete it in a later layer. Which means an attacker with access to the image can get the secret.

Pass the secret in using –build-arg

Unfortunately build arguments are also embedded in the image: an attacker can run docker history --no-trunc <yourimage> and see your secrets.

COPY the secret in to an earlier the stage in a multi-stage build

The idea here is that you do a multi-stage build. In the first stage you install the code using the secret. The second stage copies the resulting code—and then you delete the first stage.

This is secure so long as you’re careful not to push that first image, but this also makes caching much harder, so you’ll end up with slow builds.

The best existing solution: the network

Whenever you do a Docker build, your RUN steps in the Dockerfile have access to the network. And if you can talk to the network, you can get secrets over the network. The earliest example I know of that does this is Dockito Vault.

Here I will present a simpler (and simplified) version.

How it works

The method I’m going to use relies on container networking:

When you run a container, it gets its own network namespace in the kernel, with its own network interfaces and corresponding IP addresses. Containers can choose to join the network of an existing container. docker build has a --network argument that lets RUN build steps join a particular network—including that of an existing container.

That means we can run a webserver with the secrets, and then the build process can talk to that webserver to get those secrets.

An example

We use the busybox image to start a container serving a secret, and we call the resulting container secrets-server :

$ cat secret.txt gadzooks123 $ docker run --name = secrets-server --rm --volume $PWD :/files \ busybox httpd -f -p 8000 -h /files

Next, I want to build a Dockerfile that needs this secret. For demonstration purposes I will write the secret to stdout, but obviously you don’t want to do this in a real Dockerfile.

FROM busybox RUN echo "The secret is: " && \ wget -O - -q http://localhost:8000/secret.txt

And now we run the build in the network namespace of secrets-server , so we can access the webserver:

$ docker build --network = container:secrets-server . Sending build context to Docker daemon 3.072kB Step 1/2 : FROM busybox --- > 64f5d945efcc Step 2/2 : RUN echo "The secret is: " && wget -O - -q http://localhost:8000/secret.txt --- > Running in 37db7718316a The secret is: gadzooks123 Removing intermediate container 37db7718316a --- > b464bdd511f9 Successfully built b464bdd511f9

And there you have it: a build that has access to secrets in a secure way.

You will want to make sure you don’t write these secrets to disk beyond the lifetime of a single RUN line, so that they don’t get persisted in one of the image’s layers. Writing them to /dev/shm is a good way to achieve that, since that is an in-memory filesystem and never persisted in the resulting image (nor does it persist for more than the life time of the RUN line).

Avoid secret leaks

If you have passwords, keys, or tokens you need to build your Docker image, make sure you don’t leak them. Either use the newer BuildKit-based mechanisms, or use the network-based approach I covered here.