Respect ALL_PROXY during registry operations
Respect ALL_PROXY during registry operations

Use sockets.DialerFromEnvironment, as is done in other places,
to transparently support SOCKS proxy config from ALL_PROXY
environment variable.

Requires the *engine* have the ALL_PROXY env var set, which
doesn't seem ideal. Maybe it should be a CLI option somehow?

Only tested with push and a v2 registry so far. I'm happy to look
further into testing more broadly, but I wanted to get feedback on
the general idea first.

Signed-off-by: Brett Higgins <9b0bd3c592a74f73dc42aa83c8abd54c99e84f73@arbor.net>

thaJeztah/distribution,thaJeztah/distribution