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:

- `UNSET ENV ENV1 ENV2 ...` unset environment variables inherited from parent image
- `UNSET EXPOSE PORT ...`  remove exposed ports inherited from parent image
- `UNSET VOLUME PATH` remove volume mountpoints from parent image

Note: `UNSET VOLUME` 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.
I would like to see these runconfig elements as separate functions. This function is extremely long and anything that goes towards making it simpler, I am all for.