CI support stuff

Docker image

GitLab CI jobs run in a Docker image, defined here. To update that image (perhaps to install some more packages):

1. Edit .gitlab-ci/Dockerfile with the changes you want
2. Edit .gitlab-ci/run-docker.sh and bump the version in TAG
3. Run .gitlab-ci/run-docker.sh to build the new image, and launch a shell inside it
   • When you're done, exit the shell in the usual way
4. Run .gitlab-ci/run-docker.sh --push to upload the new image to the GNOME GitLab Docker registry
   • If this is the first time you‘re doing this, you’ll need to log into the registry
   • If you use 2-factor authentication on your GNOME GitLab account, you'll need to create a personal access token and use that rather than your normal password
5. Edit .gitlab-ci.yml (in the root of this repository) to use your new image