Proto3 is now required for Java, so build it including the Java runtime
since it is not on Maven.

The images seem to have been broken starting with 12e12a39, but the
removal of Maven support from grpc-java also broke them further.

SSH handling no longer uses $HOME as it does not agree with /etc/passwd
in older docker versions[1] and prevented SSH from seeing the config
file.

Some effort was also made to reduce the image sizes by removing
temporary files.

1. https://github.com/docker/docker/issues/2968

- necessary to allow seamless interop testing, as discussed earlier this week.