Deimos should not override workdir

[guenter]

Deimos always sets the workdir to /tmp/mesos-sandbox (via -w) which makes it hard to use containers that expect to be in the dir that is set via WORKDIR in Dockerfile. It would be nice if Deimos could use WORKDIR instead.

[solidsnack]

It isn't really clear that mounting the fs (where the URLs are unpacked) under WORKDIR is safe in general...but not adding -w when there is a WORKDIR seems easy enough.