The code introduced in ac20c72b was buggy, because it only checked the
existence of the APT cache directory. If two containers were started at
the same time, sometimes the code would run into a race condition; one
of the containers would update the APT package cache, the second would
assume, as the directory did exist, that an update of the package cache
was unneeded, accordingly, installing packages in the second container
failed.

This commit introduces a better check: It doesn't rely on the existence
of the directory, but uses 'find' to check the 'mtime' of the contained
list files. If there are none (in case the GitLab CI cache is empty), or
the last package update run was (more than) 14 days ago, the package
cache is updated.

To make the caching working reliable, another change on the GitLab
Runner was needed. In the config, 'cache_dir' was set to a directory on
the host and 'volumes' was accordingly extended via adding this
directory. Unfortunately, the GitLab CI docs are quite sparse in this
regard, so it took a while to figure this out.

This change was tested several times against different (force-pushed)
branches and with "cold and hot" cache.

On top, this commit clarifies the naming of the environment variables
which are used in the APT config.

Closes #316