Use pre-built Docker images for CI
Some of the execution time spent by our CI pipeline is in downloading and installing system dependencies such as SuiteSparse and conda. The git.ligo.org GitLab server provides a container registry that lets you host your own Docker images to avoid this constant rebuilding. Instead, the Docker images are built once and downloaded by the jobs in the pipeline.
In this merge request I have modified the CI pipeline to use the Docker containers built and pushed to the container registry on the new Finesse Docker repository. A CI pipeline there builds and pushes the containers to the registry using the images contained therein.
This changes the procedure for modifying upstream system dependencies. I have documented this in the developer guide.