Auto DevOps to test Docker images
Description
Auto DevOps is great in that it runs a bunch of things, including tests, automatically for your code. But Herokuish, which it depends on, is really slow. If you provide a Dockerfile, we'll use that instead, but there's no equivalent for tests. We'll still run Herokuish for tests. Is there a convention people already use for testing Docker images? There's only one cmd per Docker image, so we can't deduce what test command to run. One suggestion is to detect a Dockerfile-test, build that, and run its default cmd for the test phase. Even if this isn't widespread yet, it's a reasonable convention we could recommend and adopt, especially since there's almost no downside. If you don't have it, we'll still default to Herokuish buildpacks, but if it is present, we'll use it. It's somewhat unlikely that Dockerfile-test would exist for another reason and you wouldn't want to use it for tests. The real problem is discoverability as this would likely be buried in documentation. With dynamic guides, we could recommend creating Dockerfile and Dockerfile-test when not present. Also, then our project templates could get rid of their .gitlab-ci.yml and just provide Dockerfile and Dockerfile-test files (and Helm charts if necessary) for more optimal pipelines.