use a volume for the initrd image

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/Makefile

@@ -8,7 +8,7 @@ initrd.img: Dockerfile mkinitrd.sh repositories $(ETCFILES)
 	$(MAKE) -C packages
 	$(MAKE) -C kernel
 	cp inittab.x86_64 etc/inittab
-	docker-compose run --rm -T moby /bin/mkinitrd.sh > $@
+	docker-compose run --rm -T moby /bin/mkinitrd.sh
 
 mobylinux.iso: initrd.img Dockerfile.iso isolinux.cfg
 	docker-compose run --rm -T iso cat /tmp/output.iso > $@

alpine/docker-compose.yml

@@ -4,6 +4,8 @@ services:
     build:
       context: .
     network_mode: bridge
+    volumes:
+     - .:/mnt
   arm:
     build:
       context: .

alpine/mkinitrd.sh

@@ -65,4 +65,4 @@ printf 'docker' > /tmp/etc/hostname
 rm /tmp/bin/mkinitrd.sh
 
 cd /tmp
-find . | cpio -H newc -o
+find . | cpio -H newc -o > /mnt/initrd.img