Now, in order to set up the builder at a given remote @ commit , we
will want to make
${docker_src}/built/openwrt-builder-remote_commit.sentinel
... and then use the openwrt-builder-remote_commit image as a base.
New remotes can / should be added to openwrt-remotes.txt. Honestly
this oughta have been under the openwrt dir but, eh.
