gitlab-ci: build and deploy docs

Dunno if alpine is a good idea. It's what the gitlab docs use for most
of their examples, so that's what I've gone with... Can probably be
changed to something else if wanted.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>
1 file changed