Remove apt cache for each layer

[robin-reckmann]

Remove cached apt-related files after each build step that includes apt package installations. This reduces the size of the final docker image from 6.61GB to 5.85GB.
Note: It does NOT affect the size of the final system.img

[andiradulescu]

Wouldn't it be better to not remove /var/lib/apt at all between steps and just delete it in the last Dockerfile.agnos step? This way we can call apt update just once at the beginning e.g. in base_setup.sh.

[robin-reckmann]

In the end it's a tradeoff between docker image size and build speed. If you e.g. don't remove these files in the base_setup but only in the end, then they will still stay in the base_setup layer. Separating update and install could also lead to potential caching issues. (see here https://docs.docker.com/build/building/best-practices/ ).

[andiradulescu]

see here https://docs.docker.com/build/building/best-practices/

Very nice write up!

[adeebshihadeh]

It's not clear to me this tradeoff is worth it. Where is the individual layer size a problem?

[robin-reckmann]

This is also done in #308 in a better way. Closing.