feat(gitlab-ci): Provide additional per arch/branch "latest" tag

This patch adds a <branch slug>-latest-<arch> tag to each built image, that will eventually provide the newest built of a branch. This feature was added as it was required for optimised container builds that don't rebuild all references on every commit.

It's worth noting that the usage of the commit specific builds is still recommended for consistency reasons.

It's implemented using podman push's feature of providing an additional remote reference when pushing an image. This way, there is no re-tagging required.

References: fixes #5 (closed)