Push an abridged README to Docker Hub

[beorn7]

The full README.md exceeds the character limit for the description on Docker Hub. This commit pushes an abridged README-docker.md to Docker Hub. It contains a reference to the full README.md.

[SuperQ]

This file is synced from prometheus/prometheus, changes will be overwritten.

[SuperQ]

Maybe we should move some of the text to other documentation files?

[beorn7]

So far, having all the documentation for a relatively simple app just in README.md served us well.
Maybe it's time to move on to "real" documentation, but I don't want this to be motivated by a character limit on Docker Hub in combination with the inflexibility of our tooling.

[beorn7]

Is there some way to implement an override for the GH actions? Should we create one?

[beorn7]

Here is an idea: readme_file: 'README.md' is the default behavior anyway, so we could just leave it out. This has the additional twist that the tool will prefer a file called README-containers.md if it exists. (At least that's how I understand https://github.com/christian-korneck/docker-pushrm .) This behavior seems to be precisely tailored to the use case at hand here.

So plan would be:

1. Remove readme_file: 'README.md' from the source in prometheus/prometheus.
2. Let it propagate.
3. Rename README-docker.md in this PR to README-containers.md (and revert the other changes).

Only drawback is that it will deliver the shorter README also to Quay (which apparently allows larger descriptions), but I guess that's fine.

WDYT?

[beorn7]

@SuperQ what do you think about the idea above?

[beorn7]

Chatted with @SuperQ , and the plan seems to work.

prometheus/prometheus#14211 is merged for step 1. We have to wait until it has propagated (step 2), and then I will change this PR to implement step 3.

[beorn7]

Updated according to the new plan. @SuperQ PHAL.