Initial Dockerfile (See #1216)

Simple dockerfile which uses the instructions at
http://cellprofiler.org/linux.shtml to build an
image.
> I suppose total coolness would be if the Dockerfile installed its own commit

That would likely be a build-time step then. Rather than using the yum repository, use a `COPY` statement to move the build CP into the docker image.

> but perhaps there isn't enough magic to make that possible. Perhaps it should be RUN yum install -y cellprofiler-2.1.1?

That would certainly work for me. Shall I push that?