Add a "helper" variant image to get started with `d1h` quickly

Re #1.

derive.sh

@@ -26,3 +26,21 @@ if test $SUITE = stable; then
     docker tag  $IMAGE:$SUITE $IMAGE
     docker push $IMAGE
 fi
+
+BASE_IMAGE=$IMAGE
+IMAGE=$CI_REGISTRY_IMAGE/helper
+
+docker build \
+       --build-arg DEVUAN_IMAGE=$BASE_IMAGE \
+       --tag $IMAGE:$STAMP \
+       --file $(basename $IMAGE).df .
+
+docker push $IMAGE:$STAMP
+docker tag  $IMAGE:$STAMP $IMAGE:$ALIAS
+docker push $IMAGE:$ALIAS
+docker tag  $IMAGE:$ALIAS $IMAGE:$SUITE
+docker push $IMAGE:$SUITE
+if test $SUITE = stable; then
+    docker tag  $IMAGE:$SUITE $IMAGE
+    docker push $IMAGE
+fi

helper.df

@@ -0,0 +1,16 @@
+# helper.df -- a Devuan image with d1h to speed up package maintenance
+# Copyright (C) 2017  Olaf Meeuwissen
+#
+# License: GPL-3.0+
+
+ARG   DEVUAN_IMAGE
+FROM  ${DEVUAN_IMAGE}
+LABEL maintainer="Olaf Meeuwissen <paddy-hack@member.fsf.org>"
+
+RUN echo "deb http://packages.devuan.org/devuan/ experimental main" \
+         > /etc/apt/sources.list.d/experimental.list \
+    && apt-get update -q \
+    && DEBIAN_FRONTEND=noninteractive \
+       apt-get install -q -y \
+               -t experimental d1h \
+    && remove-apt-lists