Ensure we have the latest fnproject/dind (#687)

If we need to reissue fnproject/dind:17.12 (which fnproject/fnserver
is based upon) then let's make sure we're using the latest one
when cutting a release.

To ensure we don't accidentally use stale images lying around in
the docker cache (there probably shouldn't be *any*), call
    make clear-images
before running the build.

images/dind/preentry.sh

@@ -6,16 +6,11 @@ if [ $fsdriver == "overlay" ]; then
   fsdriver="overlay2"
 fi
 
-mkdir -p /etc/docker
-/bin/cat > /etc/docker/daemon.json <<EOF
-{
-  "mtu": $(ip link show dev $(ip route |
-                              awk '$1 == "default" { print $NF }') |
-           awk '{for (i = 1; i <= NF; i++) if ($i == "mtu") print $(i+1)}')
-}
-EOF
+mtu=$(ip link show dev $(ip route |
+                         awk '$1 == "default" { print $NF }') |
+      awk '{for (i = 1; i <= NF; i++) if ($i == "mtu") print $(i+1)}')
 
-dockerd-entrypoint.sh --storage-driver=$fsdriver &
+dockerd-entrypoint.sh --storage-driver=$fsdriver --mtu=$mtu &
 
 # give docker a few seconds
 sleep 3