Add initial scripts for image maintainers to use to auto-generate appropriate GitHub Actions

[tianon]

This (plus docker-library/official-images#7888) has been used to successfully run tests in GitHub actions across the following images:

• python: https://github.com/docker-library/python/actions/runs/89429902
• mongo: https://github.com/docker-library/mongo/actions/runs/89359532
• openjdk: https://github.com/docker-library/openjdk/actions/runs/89385581
• julia: https://github.com/docker-library/julia/actions/runs/89398688
• golang: https://github.com/docker-library/golang/actions/runs/89404277
• postgres: https://github.com/docker-library/postgres/actions/runs/89429254

I've not done much testing/thinking around how I'd use this as a non-docker-library maintainer (esp. around my personal repos which I use this pattern for maintaining, for example), but I think this gives us a solid base to move outwards from.

Most of this was made possible by https://github.blog/changelog/2020-04-15-github-actions-new-workflow-features/ -- I was hoping we could take this a step further and have the script directly generate steps: values (because that's way nicer looking, especially for builds that do multiple images; https://twitter.com/tianon/status/1253032501796147200), but the answer is no (maybe "not yet?" 🤞 😅).

[tianon]

I've not done much testing/thinking around how I'd use this as a non-docker-library maintainer (esp. around my personal repos which I use this pattern for maintaining, for example), but I think this gives us a solid base to move outwards from.

I did a little testing on this and was able to make it work reasonably transparently with BASHBREW_NAMESPACE with very minimal changes here (dd71778). 👍

[tianon]

Very successful non-docker-library PoC in tianon/dockerfiles@9f8c457 -- the result in https://github.com/tianon/dockerfiles/runs/624403625 looks about how I'd expect it to (and resulted in 53b96a6 + b22f02e here).