ENTRYPOINT is used even when other commands are specified on the "docker
run" command line. This allows running one-off commands in the docker
image (especially combined with volume binding with the host) with the
correct environment variables.
It is a bad idea to check out code into the docker image, as it will be
out-of-date. It is better to have the image just be the environment, and
let any scripts that need source check them out themselves.
This fixes#4419 in that it allows the image to build again, albeit
users would need to use wget to grab the source of the version of
protobuf they wish.
