Stop using volume and compose to build initrd

This makes remote builds work, and simplifies build.

See #469

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

alpine/Makefile

@@ -7,8 +7,8 @@ initrd.img: Dockerfile mkinitrd.sh init $(ETCFILES)
 	rm -f initrd.img
 	$(MAKE) -C packages
 	$(MAKE) -C kernel
-	docker-compose build moby
-	docker-compose run --rm -T moby /bin/mkinitrd.sh
+	docker build -t moby-initrd:build .
+	docker run --net=none --rm moby-initrd:build /bin/mkinitrd.sh > $@
 
 initrd.img.gz: initrd.img
 	cat initrd.img | gzip -9 > initrd.img.gz
@@ -66,7 +66,6 @@ azure: common
 clean:
 	rm -f initrd.img initrd.img.gz initrd-arm.img Dockerfile.armhf mobylinux.vhd mobylinux.img
 	rm -f mobylinux-bios.iso mobylinux-efi.iso mobylinux.efi
-	docker images -q alpine_moby:latest | xargs docker rmi -f || true
 	docker images -q alpine_efi:latest | xargs docker rmi -f || true
 	docker images -q alpine_bios:latest | xargs docker rmi -f || true
 	docker images -q alpine_arm:latest | xargs docker rmi -f || true

alpine/docker-compose.yml

@@ -1,11 +1,5 @@
 version: '2'
 services:
-  moby:
-    build:
-      context: .
-    network_mode: bridge
-    volumes:
-      - .:/mnt
   efi:
     cap_add:
       - sys_admin

alpine/mkinitrd.sh

@@ -35,4 +35,4 @@ printf 'moby' > /tmp/etc/hostname
 rm /tmp/bin/mkinitrd.sh
 
 cd /tmp
-find . | cpio -H newc -o > /mnt/initrd.img
+find . | cpio -H newc -o