Fix invalid syntax of github action file

[errnesto]

Okay I'm very sorry for my messy contributions :-( The github action is still broken.
I'm trying to clean it up here.

The last time. I tested the action locally with https://github.com/nektos/act where it worked fine, but the syntax for the action was still invalid.

By now I don't know if there is a good way to do this with only an action file.
So I created a entrypoint.sh file and another Dockerfile.
There it is easier to access the input variables from github.

And I added a "test" here:
https://github.com/wunderundfitzig/linkcheck-action/runs/4301401999?check_suite_focus=true

[andrenarchy]

It would be great to see this merged and in a new release as soon as possible because the action is not usable for us currently.

[errnesto]

@filiph ping :-)
I’m also happy to apply some changes to this. e.g. if you would prefer to use just one Dockerfile. Then the entrypoint.sh would just go unused in the normal case.

[filiph]

Thanks for the ping! To be honest, I don't know enough about Docker to review this. I hoped I'd have time by now to learn more, but I don't. I'll just merge it. Please let me know if there's anything else I need to do.

[tooomm]

@filiph Since your published action is sadly still not working in the GitHub Marketplace... do you mind trying to publish a new release targeting master?
All changes in recent months (like this one) that were targeted at fixing the GH Action are not yet in a published release.
It looks like you published the current version in February 2022, but targeted 2.0.20 from October 2021 (69913d9). Therefore the change in this PR is not included and distributed yet.

I created #95 and drafted a new release.