Check if stage exists in Dockerfile - docker

Question

I have a CI script that builds Dockerfiles. My plan is that unit tests should be run in a test stage in each Dockerfile, for example:
FROM alpine AS build
WORKDIR /app
COPY src .
...
FROM build AS test
RUN mvn clean test
FROM build AS package
COPY --from=build ...
So, for a given Dockerfile, I would like to check if it has a test stage and, if so, run docker build --target test .... If it doesn't have a test stage, I don't want to run docker build (which would fail).
How can I check if a Dockerfile contains a certain stage without actually building it?
I do realize this question has some XY problem vibes to it, so feel free to enlighten me. But I also think the question can be generally useful anyway.

Answer 1

I'm going to shy away from trying to parse the Dockerfile since there are a lot of ways to inject false positives or negatives. E.g.
RUN echo \
FROM base as test
or
FROM base \
as test
So instead, I'm going to favor letting docker do the hard work, and modifying the file to not fail on a missing test. This can be done by adding a test stage to a file even when it already as a test stage. Whether you want to put this at the beginning or end of the Dockerfile depends on whether you are running buildkit:
$ cat df.dup-target
FROM busybox as test
RUN exit 1
FROM busybox as test
RUN exit 0
$ DOCKER_BUILDKIT=0 docker build --target test -f df.dup-target .
Sending build context to Docker daemon 20.99kB
Step 1/2 : FROM busybox as test
---> be5888e67be6
Step 2/2 : RUN exit 1
---> Running in 9f96f42bc6d8
The command '/bin/sh -c exit 1' returned a non-zero code: 1
$ DOCKER_BUILDKIT=1 docker build --target test -f df.dup-target .
[+] Building 0.1s (6/6) FINISHED
=> [internal] load build definition from df.dup-target 0.0s
=> => transferring dockerfile: 114B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 34B 0.0s
=> [internal] load metadata for docker.io/library/busybox:latest 0.0s
=> [test 1/2] FROM docker.io/library/busybox 0.0s
=> CACHED [test 2/2] RUN exit 0 0.0s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:8129063cb183c1c1aafaf3eef0c8671e86a54f795092fa7a918145c14da3ec3b 0.0s
Then you could append the always successful test at the beginning or end, passing that modified Dockerfile to stdin for the docker build to process:
$ cat df.simple
FROM busybox as build
RUN exit 0
$ cat - df.simple <<EOF | DOCKER_BUILDKIT=1 docker build --target test -f - .
FROM busybox as test
RUN exit 0
EOF
[+] Building 0.1s (6/6) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 109B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 34B 0.0s
=> [internal] load metadata for docker.io/library/busybox:latest 0.0s
=> [test 1/2] FROM docker.io/library/busybox 0.0s
=> CACHED [test 2/2] RUN exit 0 0.0s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:8129063cb183c1c1aafaf3eef0c8671e86a54f795092fa7a918145c14da3ec3b 0.0s

Answer 2

This is a simple grep invocation:
egrep -i -q '^FROM .* AS test$' Dockerfile
You also might consider running your unit tests outside of Docker, before you start building containers. (Or, if your CI system supports running steps inside containers, use a container to get a language runtime, but not necessarily run the Dockerfile.) You'll still need a Docker-based setup to run larger integration tests, but you can run these on your built production-ready containers.