Support: --sources-dir or --source and --retrieve for --spec

Either supply a --sources-dir containing resources for spec
or supply list of tarballs using --source.

If --retrieve is passed after --spec, trigger spectool
to download resources inside the container.
Print errors from docker
Merge branch 'master' of https://github.com/norrs/docker-rpmbuild into norrs-master

Conflicts:
	rpmbuild/__init__.py
Merge branch 'norrs-master'
Making #4 compatible with home directory changes