Fix deadlock on failed dial in UDP userland proxy

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>
Merge pull request #7559 from LK4D4/fix_udp_proxy_deadlock

Fix deadlock on failed dial in UDP userland proxy
Save "COMMENT" field in Dockerfile into image content.

This will allow a user to save COMMENT data into an image, which
can later be retrieved using:

docker inspect IMAGEID

Copied the test example and some description data from Erik Hollensbe
<github@hollensbe.org>

Add support to allow docker import to populate Comment field

This patch also adds an option to docker import -m  Which will allow
base image builder to populate docker images with more comments then
just imported from FOOBAR.  This matches the docker commit -m functionality.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

Conflicts:
	docs/sources/reference/builder.md

Conflicts:
	server/image.go