Initial Dockerfile (See #1216)

Simple dockerfile which uses the instructions at
http://cellprofiler.org/linux.shtml to build an
image.
I think here, we may want versioning information so that it pulls in the latest release. I suppose total coolness would be if the Dockerfile installed its own commit, but perhaps there isn't enough magic to make that possible. Perhaps it should be `RUN yum install -y cellprofiler-2.1.1`?