So I got asked about this topic after my DevConf 2016 talk: there is a solution available on internets which describes how one can use two dockerfiles to build an image. Whole article can be found here.
What I didn’t like about the solution is that the first image outputs whole
build artifact as a tarball to standard output. To me that’s a bit hacky. Since
docker 1.8 you can
cp files and directories between containers and host.
Let’s try to do that!
All of this is because of build secrets. It may happen that you need to
authenticate with an external service when building a docker image. In order to
do that, you need to have a secret available during build. That’s a problem.
This key may leak into a final image (whether via
docker history or will be
available directly in some layer).
Here’s a solution!
Split your build process into two steps, each step represents its own dockerfile.
Authenticate with external service in order to fetch sources (use private SSH key to authenticate with GitHub so you can clone a repo) and build the project.
Get build artifacts from step 1 and install them.
Let’s do this!
First we need to write a Dockerfile which is able to fetch and build the project:
FROM fedora:23 RUN dnf install -y git # this is the private key you DON'T want to get leaked COPY id_rsa / # just for the demo; we are not using the key actually RUN git clone https://github.com/TomasTomecek/sen /project && \ cd /project && \ python3 ./setup.py build # make clean would make sense here
Let’s get the key:
cp -a ~/.ssh/id_rsa id_rsa
and don’t forget to blacklist the key in
printf "id_rsa\n" >.gitignore
docker build --tag=build-image .
We can copy the build artifact from build container now:
docker create --name=build-container build-image cat docker cp build-container:/project ./build-artifact
You are free to inspect and post-process the artifact:
ls -lha ./build-artifact
Everything is fine? If so, let’s build the final image.
docker build -f Dockerfile.release --tag=sen .
Is the key in final image?
cat ./test-if-key-is-present.sh if docker run sen test -f /id_rsa then printf "Key is in final image!\n" exit 2 else printf "Key is not in final image.\n" fi
./test-if-key-is-present.sh Key is not in final image
Whole solution is available in this GitHub repo.