This includes two logically separate changes:
- on the host, try not to require sudo when running the build scripts
- namely when interacting with the docker daemon, this requires
the unix user on the host to be part of the `docker` group
- this solves part of https://github.com/spesmilo/electrum/issues/7602
- while running inside the docker containers, do not run as root
- this means that e.g. files created in mounted folders should
no longer be owned by root on the host
- there is some code duplication involved here - not sure
how it could be deduped.
