commit 5352082045f9ced8e58ae4bec44f4cfab4bc59c3
Author: Max Rottenkolber <max@mr.gy>
Date:   Sat Sep 19 18:54:31 2015 +0200

    Add first blog entry “Building VM Images from Docker Containers”.

diff --git a/blog/build-vm-image-with-docker.mk2 b/blog/build-vm-image-with-docker.mk2
new file mode 100644
index 0000000..e607f13
--- /dev/null
+++ b/blog/build-vm-image-with-docker.mk2
@@ -0,0 +1,47 @@
+[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:
+
+#code Convert a Docker container to a raw file system image.#
+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
+#
+
+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:
+
+#code Example: Re-enable init in {ubuntu:14.04} Docker image.#
+FROM ubuntu:14.04
+
+RUN rm /usr/sbin/policy-rc.d \
+    && rm /sbin/initctl \
+    && dpkg-divert --local --rename --remove /sbin/initctl
+#
+
+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}”.