allow specifying the Docker repo for 'tested_coq_opam_versions' for use in coq-action.yml

[erikmd]

Follow-up of #57

[erikmd]

just tested here the new docker-coq-action config (albeit with the default values, i.e., no custom repo) → ✔️

[palmskog]

I think we should try to keep the semantics of tested_coq_opam_versions the same across the different CI platforms as far as possible. Unfortunately, this PR introduces a different semantics compared the Travis CI one.

Consider the following definition in meta.yml:

tested_coq_opam_versions:
- version: '8.12'
- version: '1.11.0-coq-8.11'
  repo: 'mathcomp/mathcomp'

The expected meaning is that there is one run for the image coqorg/coq:8.12 and one run for the image mathcomp/mathcomp:1.11.0-coq-8.11. However, one instead gets coqorg/coq:8.12 and coqorg/coq:1.11.0-coq-8.11.

The easiest way to fix this is probably to replace image_version with just image, and specify the whole identifier (<repo>:<version>) of each image there.

[erikmd]

@palmskog good point, I had not fully realized repo can be "local" to the version item :) I'm going to push a new commit.

[palmskog]

Thanks @erikmd, I tried the update version on one of my repos with different images and it works great. Let's merge this one now.