Rework the builder targets so we may use different remotes

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.

openwrt-builder-commit.dockerfile

@@ -1,10 +1,17 @@
 FROM openwrt-builder
 
-ARG commit
+ARG remote="origin"
+
+ARG remoteurl="https://git.laboratoryb.org/hurricos/openwrt-mirror"
+
+ARG commit="master"
 
 RUN cd ${openwrt_dir} \
-    && git pull origin master \
-    && git checkout ${commit} \
+    && ( git remote add ${remote} ${remoteurl} --tags || true )
+
+RUN cd ${openwrt_dir} \
+    && git fetch --all \
+    && git checkout ${remote}/${commit} \
     && ./scripts/feeds update -a \
     && ./scripts/feeds install -a