Add daemon flag to specify public registry mirrors

Adds support for a --registry-mirror=scheme://<host>[:port]
daemon flag. The flag may be present multiple times. If
provided, mirrors are prepended to the list of endpoints used
for image pull. Note that only mirrors of the public
index.docker.io registry are supported, and image/tag resolution
is still performed via the official index.

Docker-DCO-1.1-Signed-off-by: Tim Smith <timbot@google.com> (github: timbot)
engine.Env: comments and tests for Get()

Signed-off-by: Vincent Batts <vbatts@redhat.com>
builder/parser/dumper: Handle newlines appropriately.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder/parser: fix handling of empty strings as the only argument.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: Fix a small bug that could improperly cache.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: Fix entrypoint and cmd semantics in evaluator. Test c/o @cnf

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
evaluator: ensure entrypoint stays blank if set blank while CMD is set.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Do not populate path to prevent runconfig merge conflict; remove
autoConfig seeding of entrypoint.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
move entrypoint integration-cli test for blank entrypoint to separate function

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: Fix an error check being done at the wrong spot in run
dispatcher

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #7984 from vbatts/vbatts-engine_env_comments

engine.Env: comments and tests for Get()
Merge pull request #7806 from erikh/fix_cmd_handling_in_parser

Fix cmd and entrypoint handling in parser
Merge pull request #7202 from timbot/add-registry-mirror-flag

Add daemon flag to specify public registry mirrors