docker-registry: Add a Gentoo-based docker-registry

The requirements are installed using Portage, because that handles
non-Python requirements gracefully, and caches all the source tarballs
for later use (e.g. if PyPI is down).  Note with the 20140206
snapshot, this installs dev-python/gevent-1.0_rc3, which is based on
libev not libevent like the pre 1.0 gevents [1]:

  New version of gevent uses libev and c-ares rather than libevent and
  includes all the dependencies in the release tarball.

You may have to bump your gevent requirements (docker-registry's
requirements.txt currently pins it at 0.13.8) or install
dev-libs/libevent if you want to run the tox test suite.

Bugsnag doesn't have a Gentoo package, so I don't install it.  That
means you won't be able to use Bugsnag with this image (unless you
install it yourself), and you'll want a version of docker-registry
that includes e253550 (Merge pull request #241 from
wking/optional-bugsnag, 2014-02-11).

[1]: https://pypi.python.org/pypi/gevent/0.13.8
build.sh: Add BUILD_OPTS for custom build options

For example, to rebuild a package without using the cache:

  $ REPOS=docker-registry BUILD_OPTS=--no-cache ./build.sh

which lets you update your image after the upstream repository has
changed.