Adds the GCB build yaml for controlling the build, and updates the test script
to work in both the GCB environment and on a local dev machine.
Also changes the centos buildbox to leave the default user as root. When
GCB mounts the workspace into the container, the source code is owned
by root, and there is no way to change this. This means that the build will
fail when the non-root user specified in the build image attempt to write files
into the workspace. Setting the root user fixes this.
See-Also: #15186
