util/release: Support custom Docker registry & user

Signed-off-by: Lewis Marshall <lewis@lmars.net>
pinkerton: Support BasicAuth with Docker endpoints

Signed-off-by: Lewis Marshall <lewis@lmars.net>
website: Document releasing Flynn

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #382 from flynn/custom-docker-repository

Custom Docker registry / user