Taking a whack at a Docker image

The install process is ugly; there must be a better way.  Maybe
instead of letting `docker build` compile everything, it depends on
the tarball (or an RPM) already existing in the `target` directory?

To build:

    docker build -t blalor/hook-hub:${VERSION} .

To run, assuming there's already a container named "redis" running:

    docker run -d -P -link redis:db blalor/hook-hub:${VERSION}