The build arguments `VCS_REF` and `VCS_VER` were renamed and given
proper values according to their names.
1. `VCS_REVISION` holds the current SHA sum of the (git) HEAD pointer
2. `VCS_VERSION` now holds the contents of the `VERSION` file, i.e. a
semver version tag (one can now inspect the image and find a proper
version tag in the `org.opencontainers.image.version` label)
The build arguments were given defaults in order to allow the
`generic_build` and `generic_test` workflows to omit them (as they are
not need there anyways). When publishing images, this is fina as the
cache will rebuild almost all of the image except the last few layers
which are `LABEL`s anyways.
Mew re-usable workflows are introduced to handle building, testing and publishing the container
image in a uniform and easy way. Now, the `scheduled_builds`, `default_on_push`
and a part of the `test_merge_requests` workflow can use the same code
for building, testing and publishing the container images. This is DRY.
Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>
