Add section on managing docker disk usage

[rdnfn]

Problem

Beobench is able to use a lot of disk space. We should add a section in the docs that elaborates how to view the docker related disk usage and how to remove older build cache entries.

Resources:

• https://stackoverflow.com/questions/66911920/get-docker-container-disk-usage-as-percentage
• https://stackoverflow.com/questions/65405562/is-there-a-way-to-clean-docker-build-cache

Potential Solution

Add info to docs.