ci: Replicate the CI image scripts from GLib

They are easier to use and document, and allow us to add new base images
in the future without making a mess.

.gitlab-ci/README.md

@@ -0,0 +1,38 @@
+## GTK CI infrastructure
+
+GTK uses different CI images depending on platform and jobs.
+
+The CI images are Docker containers, generated either using `docker` or
+`podman`, and pushed to the GitLab [container registry][registry].
+
+Each Docker image has a tag composed of two parts:
+
+ - `${image}`: the base image for a given platform, like "fedora" or
+   "debian-stable"
+ - `${number}`: an incremental version number, or `latest`
+
+See the [container registry][registry] for the available images for each
+branch, as well as their available versions.
+
+### Checklist for Updating a CI image
+
+ - [ ] Update the `${image}.Dockerfile` file with the dependencies
+ - [ ] Run `./run-docker.sh build --base ${image} --base-version ${number}`
+ - [ ] Run `./run-docker.sh push --base ${image} --base-version ${number}`
+   once the Docker image is built; you may need to log in by using
+   `docker login` or `podman login`
+ - [ ] Update the `image` keys in the `.gitlab-ci.yml` file with the new
+   image tag
+ - [ ] Open a merge request with your changes and let it run
+
+### Checklist for Adding a new CI image
+
+ - [ ] Write a new `${image}.Dockerfile` with the instructions to set up
+   a build environment
+ - [ ] Add the `pip3 install meson` incantation
+ - [ ] Run `./run-docker.sh build --base ${image} --base-version ${number}`
+ - [ ] Run `./run-docker.sh push --base ${image} --base-version ${number}`
+ - [ ] Add the new job to `.gitlab-ci.yml` referencing the image
+ - [ ] Open a merge request with your changes and let it run
+
+[registry]: https://gitlab.gnome.org/GNOME/gtk/container_registry