BUILDING VM IMAGES FROM DOCKER CONTAINERS

                                             Max Rottenkolber <max@mr.gy>
                                              Saturday, 19 September 2015

   Dockerfiles (https://docs.docker.com/reference/builder/) are a
   convenient way to bootstrap user space file systems. The other day I
   was wondering how I could turn Docker containers into raw file system
   images that could, for instance, be run with QEMU (http://qemu.org).¹
   Maybe, this post will come in handy for others attempting to do the
   same.

   Basically you can make a copy of a Docker container's file system
   using “docker export”, which you can then write to a loop device:

     docker build -t <YOUR-IMAGE> ...
     docker create --name=<YOUR-CONTAINER> <YOUR-IMAGE>
     dd if=/dev/zero of=disk.img bs=1 count=0 seek=1G
     mkfs.ext2 -F disk.img
     sudo mount -o loop disk.img /mnt
     docker export <YOUR-CONTAINER> | sudo tar x -C /mnt
     sudo umount /mnt

       Convert a Docker container to a raw file system image.

   So far so good. You can now go ahead and attempt to run “disk.img”
   with a suitable kernel inside QEMU, but chances are that your image
   will not boot properly. This is because off-the-shelf Docker images
   usually come modified with some “Dockerisms” such as the respective
   OS's init system being prevented from running.² To run a container in
   QEMU we need to undo some of the Docker specific changes to get it to
   boot.

   Taking the official “ubuntu:14.04” image as an example, you can see
   how it disables “/sbin/initctl” in its Dockerfile
   (https://github.com/tianon/docker-brew-ubuntu-core/blob/6dba3ee12ff996640d1043139d5abf8c744862e2/trusty/Dockerfile#L4).
   To get an image with a working init, we can undo the changes in a
   Dockefile based on the “ubuntu:14.04” image:

     FROM ubuntu:14.04
     
     RUN rm /usr/sbin/policy-rc.d \
         && rm /sbin/initctl \
         && dpkg-divert --local --rename --remove /sbin/initctl

       Example: Re-enable init in ubuntu:14.04 Docker image.

   Now we can apply the steps illustrated above to get a Trusty Tahr user
   space that will run nicely in QEMU.

 * 1. The VM images derived from Docker containers do not contain a
   kernel. A kernel can be built separately and specified using QEMU's
   “-kernel” option.

 * 2. Because you usually do not need the init system when running a
   container using “docker run”.