Fix proxy settings in contrib/docker so that builds work both on the Internet and within Intel (#1088)

* Added conditional code to only set proxies when hostname appears to be on an Intel network (.intel.com)

* Replaced Intel-network-conditional proxy setting code with a system that checks for existence of http_proxy and https_proxy, like the Makefile does

* Applied fix for NGRAPH-1862, as I ran into NVidia dockerfile hangs.  Temporarily use --no-cache with docker-build, to check that proxy fix really works (and we don't get a cached version).

* Restored an original line I accidentally deleted.

* Remove --no-cache, which I had added for thorough testing.