Testing Docker images with Autotest
For awhile now, I’ve had a docker image which blindly gets rebuilt via dockerhub every time the upstream image changes. As the docker image I’m building just pulls bits and pieces from other sources, I didn’t really have anything to unit test as part of the CI process. Since it was working fine at the time, I just left it.
Fast-forward a few years and sure enough, one of the things I bundle into the image shipped a broken version. The docker build still succeeded, but the final image didn’t work as expected. Rather than let that happen again, I thought I should find a testing solution.
Docker’s Autotest looked like a good fit. It can run tests on new commits, PRs and crucially on upstream image changes. However, I was a bit confused how I could use it without bloating my image with tests embedded inside. Turns out it’s really simple using a multi-stage build.
Suppose we have the following trivial docker image, where we simploy add a dummy
example-appto the alpine image.
We can turn it into a multi-stage image with a testing layer.
First we build the regular image and label it
release. We then build a testing layer using release as a base and add in our tests. Here we just copy in a directory containing a bash script of arbitrary tests. The only requirement is that it exits with a
0when tests pass, or anything else when they fail.
Finally we call
FROM releaseto ensure that on a regular build, we only build the app without the testing layer.
For Autotest to work, we need to create a
docker-compose.test.ymlfile with a
We set the target to
testso that it builds and runs the test layer from our image. We can test it locally (with a deliberate test failure):
Now that it works locally we just need to enable Autotest in the Dockerhub settings.
Full example repository: Autotest-example.