By applying `latest` tags as soon as the relevant image has been built
we end up with a quite large time window where they are all quite out
of sync: in particular, the tag is applied to the `base` image as soon
as it is ready, before all the depending images get built.

On top of that, if depending images fail, the `latest` tags will stay
out of sync.

This can be quite confusing for users, so let's apply the `latest` tag
only after all the previous stages have completed.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>