Extract apt cache for re-use in next build

[robin-reckmann]

Downloading apt packages every time takes a considerable amount of time (This varies based on individual connection speeds). Here we extract the downloaded package indizes as well as the package cache itself and store it outside of the docker image in the ./cache directory. The build script is set up to reload this folder on the next build and re-use the package files. In CI the folder is mounted to the namespace cache volume for re-use in other runs.
Additionally apt workflows are updated so that only a single apt update in the beginning is executed and caches are reused throughout the build. This leads to a larger docker image size (due to multiple versions of the caches in the different layers), but has no influence on the final agnos image size.

[robin-reckmann]

Potential downside is that docker cache is busted more often due to changes from even other PRs to the cache. With the high download speeds in the CI this doesnt seem to be that beneficial.