Omit publishing latest/release tags on pre-releases

[ondrej-fabry]

The release process currently tags and then publishes images for tags as following:

=> new tag v21.06.0 is pushed and release process starts:

1. image is built with tag :21.06.0 for this specific tag
2. then rolling release image is tagged :21.06
3. and finally the :latest tag is updated to this release

This process is okay for the actual releases, but it should be fixed to skip step 2 and 3 for pre-releases: e.g. v21.06.1-rc1, or simply any version with suffix -*. This is because we do not want to update rolling release image or latest image tags for these pre-releases.