Use separate cache for apt

[robin-reckmann]

Move apt stuff to separate docker cache. This cache is reusable even if the Docker cache of the step itself breaks.
Also reduces the size of the Docker image, as this cache folder is not included anymore.

[robin-reckmann]

apt install step does not have to download packages, but uses cache instead:

#10 73.20 0 upgraded, 457 newly installed, 1 to remove and 0 not upgraded.
#10 73.20 Need to get 0 B/166 MB of archives.
#10 73.20 After this operation, 727 MB of additional disk space will be used.

[robin-reckmann]

Runners lock each other. Probably not the way to go in CI.

[robin-reckmann]

Go with this for local execution and use namespace cache volumes for CI instead.