Fix dependencies between command types

[markfeathers]

If we have a docker task that depends on 2 host tasks, it will not get the implicit dependency to DS_DOCKER_READY since the task already has dependencies. See the default asks in common/build.py.

This was found when adding the ZPU support where it needs host actions to install the task, but relies on no other docker tasks. By hand, that one is adding "DS_DOCKER_READY", but that should be removed once proven to correctly be implied by the docker execution environment.