Clean up after running apt-get install

Clean up data left over after runnning apt-get update and apt-get
install to make the Docker image delta smaller.
Provide custom entry point instead of bash

Use a custom entry point instead of bash so that we can make the default
user experience a little bit better. The user can still use the
--entrypoint argument to switch back to the bash shell.
vars: provide a more user friendly help
genconfig: provide more user friendly help