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