Docker to build Linux packages

[kirsle]

Adds a make rpm command that will use Docker to create RPM packages for Fedora.

This branch is an experiment to run a "poor man's CI" and build packages without needing a full Jenkins setup or anything. Eventually want to add Ubuntu and/or Debian images too.

1. make rpm runs docker/build.sh which runs the Docker build/run commands.
2. make docker.rpm runs docker/package.sh from inside the Docker container to actually run the rpmbuild steps and export them to a bind mount on the host. It safely does nothing if you run it outside the Docker environment.