ci: lint file names

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/mage2docker/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently, we have both yaml and yml in the codebase. While yaml is apparently more correct, most of the docker ecosystem uses yml. This is what happens when you don't mandate a convention upfront.

Fixes: #62 #63

What is the new behavior?

We only allow yml files.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

If you have a compose file with yaml, you should change it to yml

Other information