ci: do not run test container generation on forked repos by default

[Henrik66]

Container GitHub Action runs daily for dracutdevs developers. Every time dracut repo get's forked this daily github action runs on all forked repo's.

Some people know (and remember) to disable this action manually, but for the most this is just wasting significant resources as currently there is at east 300+ forked version of dracut on Github.

As suggested by @mwilck at #1865 (comment)

The workflow should be made conditional in some way.

Checklist

• I have tested it locally
• I have reviewed and updated any documentation if relevant
• I am providing new code and test(s) for it

Fixes #1865

[LaszloGombos]

This is a good idea, but perhaps we should allow a way for a fork to opt-in running this action without forking the action file.
Perhaps we can use https://docs.github.com/en/actions/learn-github-actions/variables to opt-in for the action (on forked repo's also).

[LaszloGombos]

Thanks.