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 <brhiggins@arbor.net>
207027087e