Building VM Images from Docker Containers

Max Rottenkolber <>

Dockerfiles 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.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. 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.