Exclude 'MESOS_' from Dockerfile variables

The `MESOS_VERSION` environment variable is a synonym for the mesos
`--version` command-line option.  Stripping `MESOS_` from all of our
variable names ensures that there won't be any collisions.

Closes #9
Merge branch 'Spike'

* Spike:
  Exclude 'MESOS_' from Dockerfile variables