Notice about missing support for Auto Test when using .buildpacks file

I had a hard time while testing out the Auto Dev Ops feature. We're a PHP shop and we're testing our stuff with phpunit. The official php buildpack doesn't support the testing feature (bin/test + bin/test-compile are missing) as of now (but as we'd like to move things forward, we provided a merge request for it).

I also learned from the documentation, that there are two ways to make the build pipeline use a custom buildpack (because, yes, we forked Heroku's official php buildpack as we really wanted to use the Auto Test feature):

• Using the project variable BUILDPACK_URL (that worked flawlessly) and
• by providing a .buildpacks file.

I took the .buildpacks road, because I like the idea of having everything CI-relevant within the repository without the necessity to setup project variables and stuff using a web UI.

It was quite hard to figure out the origin behind the idea of the .buildpacks file, but I finally came to the conclusion, that this feature is provided by Heroku's multi buildpack. And again, that buildpack doesn't support tests because of the missing files bin/test and bin/test-compile. So that brought me nowhere, back to step one, using project variables.

It could have saved me quite a lot of time doing research, when Gitlab's Auto Dev Ops documentation would have warned me, that the Auto Test feature isn't available, when I use the .buildpacks file to provide a custom buildpack.

I think other developers will run into this too, so I propose a documentation update.