Add Dockerfile instructions to reset properties inherited from parent image

This is an attemp to fix https://github.com/docker/docker/issues/3465. I've been working on this yesterday and figure that the design can be reviewed at this stage.

New added instructions are:

- `RMENV ENV1 ENV2 ...` unset environment variables inherited from parent image
- `RMEXPOSE PORT ...`  remove exposed ports inherited from parent image
- `RMVOLUME PATH` remove volume mountpoints from parent image

Note: `RMVOLUME` has the same bug that is fixed in https://github.com/docker/docker/pull/7994. Hence this should be rebased/fixed after that pull request is merged.