Integrate xfce-build container build/creation
Description
This MR implements the first iteration of an official xfce-build container deployed to https://hub.docker.com/repository/docker/xfce/xfce-build.
This also includes a base template gitlab-ci.yml that can be used by other projects that want to depend on this.
Additional Details
Before merging, add needed credentials to the environment variables for xfce4-dev-tools:
Links
Related to https://gitlab.xfce.org/infra/gitlab.xfce.org/-/issues/30
Edited by Jason Yavorska