Added Docker support (#40)

* Added Docker support

- replaced necessity for devicemapper (through kpartx) by using parted and
  losetup with offsets
- added Dockerfile
- added dependency for parted and grep
- added hints to README.md
- common: loop through unmounts, fix shellcheck warnings

* stage2: use debconf instead of console-setup patch. Fixes #41

export-image/prerun.sh

@@ -34,9 +34,21 @@ p
 w
 EOF
 
-LOOP_DEV=`kpartx -asv ${IMG_FILE} | grep -E -o -m1 'loop[[:digit:]]+' | head -n 1`
-BOOT_DEV=/dev/mapper/${LOOP_DEV}p1
-ROOT_DEV=/dev/mapper/${LOOP_DEV}p2
+PARTED_OUT=$(parted -s ${IMG_FILE} unit b print)
+BOOT_OFFSET=$(echo "$PARTED_OUT" | grep -e '^ 1'| xargs echo -n \
+| cut -d" " -f 2 | tr -d B)
+BOOT_LENGTH=$(echo "$PARTED_OUT" | grep -e '^ 1'| xargs echo -n \
+| cut -d" " -f 4 | tr -d B)
+
+ROOT_OFFSET=$(echo "$PARTED_OUT" | grep -e '^ 2'| xargs echo -n \
+| cut -d" " -f 2 | tr -d B)
+ROOT_LENGTH=$(echo "$PARTED_OUT" | grep -e '^ 2'| xargs echo -n \
+| cut -d" " -f 4 | tr -d B)
+
+BOOT_DEV=$(losetup --show -f -o ${BOOT_OFFSET} --sizelimit ${BOOT_LENGTH} ${IMG_FILE})
+ROOT_DEV=$(losetup --show -f -o ${ROOT_OFFSET} --sizelimit ${ROOT_LENGTH} ${IMG_FILE})
+echo "/boot: offset $BOOT_OFFSET, length $BOOT_LENGTH"
+echo "/:     offset $ROOT_OFFSET, length $ROOT_LENGTH"
 
 mkdosfs -n boot -S 512 -s 16 -v $BOOT_DEV > /dev/null
 mkfs.ext4 -O ^huge_file $ROOT_DEV > /dev/null