Handy Search Engine for Docker Hub Images

http://soaphub.org/imagehub/ provides a nice way to search Docker Hub. You can search for strings in name spaces, and the result also displays what architectures the image is backed by.

Public base images

https://registry.hub.docker.com/search?q=s390x shows base images available on Docker's public registry. Simply "docker pull <image>" to download them.
If you need to use a proxy, you can set the environment variable http_proxy (set it to http://proxy-address:port/) when starting the docker daemon.