gitlab-ci: Use prebuilt containers

This image has all build dependencies to speed up build and tests.
Copied from
phosh (https://gitlab.gnome.org/World/Phosh/phosh/-/commit/d8b2e5651b837adb6698c9868a9d278ef7929400)
with minor modifications for Calls.

Use

 .gitlab-ci/run-docker.sh --base debian build --version 0.0.<date>
 .gitlab-ci/run-docker.sh --base debian push --version 0.0.<date>

to update.

.gitlab-ci/README.md

@@ -0,0 +1,12 @@
+### Checklist for Updating the Docker Images
+
+ - [ ] Update the `${image}.Dockerfile` file with the dependencies
+ - [ ] Run `./run-docker.sh build --base ${image} --version ${number}`
+ - [ ] Run `./run-docker.sh push --base ${image} --version ${number}`
+   once the Docker image is built; you may need to log in by using
+   `docker login` or `podman login` like
+   podman login -u <user> -p <token> registry.gitlab.gnome.org/world/phosh/phosh
+   See https://docs.gitlab.com/ee/user/packages/container_registry/
+ - [ ] 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