Not possible to run two instances of Docker-based build simultaneously

[jhfrontz]

[This is similar to #184 except that the collision is on the name of the container produced by make-base-vm]

If you simultaneously start two different Docker-based instances of gbuild using the same make-base-vm target name (but running from different working directories), the slower/second one will call stop-target and will stop/remove the first/faster instance's container. This seems to be because (unlike with the KVM or VBOX variants) the targets (containers) are not uniquely named/identified.