Move Dockerfile GOPATH from /root/go to /usr/local/go
Move Dockerfile GOPATH from /root/go to /usr/local/go

- Concourse runs as the vcap user

cloudfoundry/cf-riak-cs-release,cloudfoundry/cf-riak-cs-release,cloudfoundry/cf-riak-cs-release,cloudfoundry/cf-riak-cs-release