Allow built images to be squash to scratch.
Squashing does not destroy any images or layers, and preserves the
build cache.
Introduce a new CLI argument --squash to docker build
Introduce a new param to the build API endpoint `squash`
Once the build is complete, docker creates a new image loading the diffs
from each layer into a single new layer and references all the parent's
layers.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
362369b4bb