Docker Support

It would be very helpful if some more reproduction steps could be provided. Perhaps even a link on documentation for how to pull and image from a registry, modify it, push the modified image, and tag the new image.

The bug is here:

https://github.com/pulp/pulp_docker/blob/17e0208d/plugins/pulp_docker/plugins/models.py#L248

We need the update call to also set the _last_updated attribute of the unit.

I suspect a reproducer would go like this:

1. build a simple docker image with a dockerfile that has something you can change. Perhaps it's just this:

FROM: busybox
ADD stuff.txt /stuff.txt

2. push that image to a registry
3. sync that repo with pulp
4. note the _last_updated timestamp on the tag unit
5. change the contents of "stuff.txt" and re-build the image
6. push the image to the same regsitry
7. sync that repo with pulp
8. note that the _last_updated timestamp on the tag unit did not change, but the value for it's manifest_digest attribute did.

Steps to verify:

1. build a simple docker image with a dockerfile that has something you can change. Perhaps it's just this:

FROM: busybox
ADD stuff.txt /stuff.txt

2. push that image to a registry
3. sync that repo with pulp
4. note the _last_updated timestamp on the tag unit and manifest_digest
5. change the contents of "stuff.txt" and re-build the image
6. push the image to the same regsitry
7. sync that repo with pulp
8. confirm that the _last_updated timestamp on the tag *did* change, also the value for it's manifest_digest attribute.
9. re-sync the repo
10. confirm that _last_updated did not changed, manifest_digest also should have stayed the same.