Fix Dockerfile for new config locations

I ran the Trusted Build service on a fork of `dotcloud:master` but the build failed with
`cp: cannot stat '/docker-registry/config_sample.yml': No such file or directory
Error build: The command [/bin/sh -c cp --no-clobber /docker-registry/config_sample.yml /docker-registry/config.yml] returned a non-zero code: 1`