Add docker support to mesos-execute

The fix for this is already committed at https://reviews.apache.org/r/24808/.

I'm creating this ticket to track that this patch gets included in 0.20.1 release, since apparently Singularity framework depends on this patch to work with Docker !?!? https://groups.google.com/forum/#!topic/singularity-users/GzzswbpI92E

tnachen: Can you confirm if this has to be included in 0.20.1?