ci: docker: deploy automatically only on default branch and tags

ci: docker: deploy automatically only on default branch and tags

In commit fecd899e8189 ("docker: deploy only manually and on
master/tags") we've introduced manual deployment of the Docker images,
but this is unneeded additional manual step, so rather prefer automatic
deployment. While at it, convert to usage of `rules:` as `only:` is
deprecated and `rules:` are preferred.

Signed-off-by: Petr Štetiar <[email protected]>