* put together a docker container that can use host gpg-agent forwarded over ssh.
* use gpg-agent forwarding container on OS X and directly forward the agent on Linux
* clean up the release container on exit
* use docker mounts instead of the deprecated volume syntax
* use image names within our project namespace
* update README to walk through running on GCE with gpg-agent forwarding
closes#1620
Signed-off-by: Matt Foley <mattf@apache.org>
Signed-off-by: Nick Dimiduk <ndimiduk@apache.org>
Sean Busbey