This commit re-adds the rolling `daily-testing-only` tag in addition to
the unique daily tag `daily-<YYYYmmdd>`.
A rolling `master` build was proposed but declined because this would
put an unnecessary burden on the build system (each run takes ~45 min)
To avoid leaking any sensitive information like Docker Hub credentials
because of compromised actions repositories, we use our own, vendored
actions for all steps that potentially touch sensitive information.
To enable building docker images for ARM64 platforms as well,
we just need to specify the desired target platforms and the Docker
Buildx service will do the job for us (provided the base images support
the given platforms, which is the case for golang).
We add a GitHub workflow that is triggered whenever a new version tag is
pushed. It will trigger a docker image build for that version and
automatically push it to the specified repo.
