Add a formatting container

This is based on the code we used for Docker Editions, and
will format an external drive, to be used for example for
`/var` for Docker image persistence.

It does not `mount` the drive yet, as splitting format and mount
gives better modularity.

Example yaml fragment:
```
  - name: format
    image: "mobylinux/format:097d4f22b20f976b1f89d8f0b8a5d074d35b856c"
    binds:
     - /dev:/dev
    capabilities:
     - CAP_SYS_ADMIN
     - CAP_MKNOD
```

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

pkg/format/Dockerfile

@@ -0,0 +1,13 @@
+FROM alpine:3.5
+
+RUN \
+  apk update && apk upgrade -a && \
+  apk add --no-cache \
+  e2fsprogs \
+  e2fsprogs-extra \
+  jq \
+  sfdisk \
+  && true
+
+COPY . ./
+CMD ["/bin/sh", "/format.sh"]

pkg/format/Makefile

@@ -0,0 +1,29 @@
+.PHONY: tag push
+
+BASE=alpine:3.5
+IMAGE=format
+
+default: push
+
+hash: Dockerfile format.sh
+	DOCKER_CONTENT_TRUST=1 docker pull $(BASE)
+	tar cf - $^ | docker build --no-cache -t $(IMAGE):build -
+	docker run --rm --entrypoint /bin/sh $(IMAGE):build -c "cat $^ /lib/apk/db/installed | sha1sum" | sed 's/ .*//' > $@
+
+push: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		(docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash) && \
+		 docker push mobylinux/$(IMAGE):$(shell cat hash))
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+tag: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash)
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+clean:
+	rm -f hash
+
+.DELETE_ON_ERROR:

pkg/format/format.sh

@@ -0,0 +1,113 @@
+#!/bin/sh
+
+# this script assumes anything on the disk can be removed if corrupted
+# other use cases may need different scripts.
+
+# currently only supports ext4 but should be expanded
+
+do_fsck()
+{
+	# preen
+	/sbin/e2fsck -p $*
+	EXIT_CODE=$?
+	# exit code 1 is errors corrected
+	[ "${EXIT_CODE}" -eq 1 ] && EXIT_CODE=0
+	# exit code 2 or 3 means need to reboot
+	[ "${EXIT_CODE}" -eq 2 -o "${EXIT_CODE}" -eq 3 ] && /sbin/reboot
+	# exit code 4 or over is fatal
+	[ "${EXIT_CODE}" -lt 4 ] && return "${EXIT_CODE}"
+
+	# try harder
+	/sbin/e2fsck -y $*
+	# exit code 1 is errors corrected
+	[ "${EXIT_CODE}" -eq 1 ] && EXIT_CODE=0
+	# exit code 2 or 3 means need to reboot
+	[ "${EXIT_CODE}" -eq 2 -o "${EXIT_CODE}" -eq 3 ] && /sbin/reboot
+	# exit code 4 or over is fatal
+	[ "${EXIT_CODE}" -ge 4 ] && printf "Filesystem unrecoverably corrupted, will reformat\n"
+
+	return "${EXIT_CODE}"
+}
+
+do_fsck_extend_mount()
+{
+	DRIVE="$1"
+	DATA="$2"
+
+	do_fsck "$DATA" || return 1
+
+	# only try to extend if there is a single partition and free space
+	PARTITIONS=$(sfdisk -J "$DRIVE" | jq '.partitiontable.partitions | length')
+
+	if [ "$PARTITIONS" -eq 1 ] && \
+		sfdisk -F "$DRIVE" | grep -q 'Unpartitioned space' &&
+		! sfdisk -F "$DRIVE" | grep -q '0 B, 0 bytes, 0 sectors'
+	then
+		SPACE=$(sfdisk -F "$DRIVE" | grep 'Unpartitioned space')
+		printf "Resizing disk partition: $SPACE\n"
+
+		START=$(sfdisk -J "$DRIVE" | jq -e '.partitiontable.partitions | map(select(.type=="83")) | .[0].start')
+
+		sfdisk -q --delete "$DRIVE" 2> /dev/null
+		echo "${START},,83;" | sfdisk -q "$DRIVE"
+
+		# set bootable flag
+		sfdisk -A "$DRIVE" 1
+
+		# update status
+		blockdev --rereadpt $diskdev 2> /dev/null
+		mdev -s
+
+		# wait for device
+		for i in $(seq 1 50); do test -b "$DATA" && break || sleep .1; mdev -s; done
+
+		# resize2fs fails unless we use -f here
+		do_fsck -f "$DATA" || return 1
+		resize2fs "$DATA"
+
+		do_fsck "$DATA" || return 1
+	fi
+}
+
+do_mkfs()
+{
+	diskdev="$1"
+
+	# new disks does not have an DOS signature in sector 0
+	# this makes sfdisk complain. We can workaround this by letting
+	# fdisk create that DOS signature, by just do a "w", a write.
+	# http://bugs.alpinelinux.org/issues/145
+	echo "w" | fdisk $diskdev >/dev/null
+
+	# format one large partition
+	echo ";" | sfdisk --quiet $diskdev
+
+	# update status
+	blockdev --rereadpt $diskdev 2> /dev/null
+	mdev -s
+
+	FSOPTS="-O resize_inode,has_journal,extent,huge_file,flex_bg,uninit_bg,64bit,dir_nlink,extra_isize"
+
+	mkfs.ext4 -q -F $FSOPTS ${diskdev}1
+}
+
+# TODO fix for multiple disks, cdroms etc
+DEV="$(find /dev -maxdepth 1 -type b ! -name 'loop*' | grep -v '[0-9]$' | sed 's@.*/dev/@@' | sort | head -1 )"
+
+[ -z "${DEV}" ] && exit 1
+
+DRIVE="/dev/${DEV}"
+
+# see if it has a partition table already
+if sfdisk -d "${DRIVE}" >/dev/null 2>/dev/null
+then
+	DATA=$(sfdisk -J "$DRIVE" | jq -e -r '.partitiontable.partitions | map(select(.type=="83")) | .[0].node')
+	if [ $? -eq 0 ]
+	then
+		do_fsck_extend_mount "$DRIVE" "$DATA" || do_mkfs "$DRIVE"
+	else
+		do_mkfs "$DRIVE"
+	fi
+else
+	do_mkfs "$DRIVE"
+fi