Fix Docker related issues for gitian-build.py
Fix Docker related issues for gitian-build.py

The Docker does not depend on apt-cacher-ng package.
Do not try to install the Docker if docker.service is detected on the
system (e.g., the Docker was installed manually).
Also small style corrections were applied.


Former-commit-id: 01a92816620513b6ac55cd964f6fe641bb53e4fe
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin