Add ENTRYPOINT.

As discussed in #102. You can simply `docker run -i -t panoptes`, or for example `docker run -i -t panoptes production`.

This does correctly attempt to start the rails server, but it currently fails with a syntax error which I assume is a separate issue.

    SyntaxError: /usr/local/lib/ruby/gems/shared/gems/cellect-client-0.0.7/lib/cellect/client/connection.rb:17: syntax error, unexpected tLABEL
          def add_subject(id, workflow_id: workflow_id, group_id: nil, priority: nil)