Improve systemd config for Raspbian

Signed-off-by: Dieter Reuter <dieter.reuter@me.com>

hack/install.sh

@@ -389,10 +389,9 @@ do_install() {
 			}
 
 			if [ "$lsb_dist" = "raspbian" ]; then
-				# Create Raspbian specific systemd unit file, use overlay by default
-				( set -x; $sh_c "mkdir -p /etc/systemd/system" )
-				( set -x; $sh_c "$curl https://raw.githubusercontent.com/docker/docker/master/contrib/init/systemd/docker.service > /etc/systemd/system/docker.service" )
-				( set -x; $sh_c "sed -i 's/dockerd/dockerd --storage-driver overlay/' /etc/systemd/system/docker.service" )
+				# Create Raspbian specific systemd drop-in file, use overlay by default
+				( set -x; $sh_c "mkdir -p /etc/systemd/system/docker.service.d" )
+				( set -x; $sh_c "echo '[Service]\nExecStart=\nExecStart=/usr/bin/dockerd --storage-driver overlay -H fd://' > /etc/systemd/system/docker.service.d/overlay.conf" )
 			else
 				# aufs is preferred over devicemapper; try to ensure the driver is available.
 				if ! grep -q aufs /proc/filesystems && ! $sh_c 'modprobe aufs'; then