[18.09] backport adding support for "registry-mirrors" and "insecure-registries" to buildkit

[AntaresS]

- What I did

• backported moby PR add support for "registry-mirrors" and "insecure-registries" to buildkit moby/moby#37852
  • cherry-picked commits 308701f
  • cherry-picked commits 171d51c
  • clean cherry-pick; no conflicts

- How I did it

> gcb cherry-37852 upstream/18.09
> git cherry-pick -x 171d51c861b479af8798fbe7c51906bcecdffea0 308701fac6f716160283ca0868da59aba2ea7eb5
[cherry-37852 a72752b2f] add support of registry-mirrors and insecure-registries to buildkit
 Date: Fri Sep 14 16:07:16 2018 -0700
 5 files changed, 73 insertions(+), 6 deletions(-)
[cherry-37852 b48afc216] update vendor
 Date: Thu Sep 13 17:44:22 2018 -0700
 5 files changed, 58 insertions(+), 12 deletions(-)
 create mode 100644 vendor/github.com/moby/buildkit/util/resolver/resolver.go

Signed-off-by: Anda Xu anda.xu@docker.com

[AntaresS]

cc @andrewhsu @thaJeztah

[AntaresS]

@andrewhsu all green !

[AntaresS]

closed in favor of a mega backport #59