* contrib: set the working directory to /src/dash to allow for cloning gitian dependencies
* contrib: place the home directory inside /home instead of root
* contrib: add notes about sharing ccache across the network
* contrib: chown based on the (u/g)id env vars instead of the associated username
* contrib: reduce layer count by reducing run invocations
* contrib: develop container cleanup and maintenance
- add apt-cacher-ng, gpg, lsb-release, screen as a package dependencies
- reorder packages in alphabetical order
- correct documentation
- create and add user to the docker group to satisfy Gitian's needs
- reduce the number of RUN calls to reduce layer count
