CI: Add configure cache for Debian job

Adds a cache for the autoconf configure script, using the --cache-file option and the GitLab CI caching mechanism. As the cache is not invalidated by autoconf in any way, we use GitLabs cache keys to invalidate it once the configure.ac file changes or if the docker image name changes. Additionally the job name is included in the cache key to ensure the cache is only used for this job and not shared with others, in case we add it to other jobs in the future.