We like containers and we like Pi computers. Containers running on our Pi’s would be like “double like” and although “double like” does not exist in the real word^* running Docker on Pi is still possible.

Good (though little bit outdated) instruction on how to install docker on your board can be found in the first part of this article. Short copy-paste is here:

$ sudo apt-get install apt-transport-https ca-certificates software-properties-common -y
$ curl -fsSL get.docker.com -o get-docker.sh && sh get-docker.sh
$ sudo usermod -aG docker pi
$ sudo reboot
$ sudo systemctl start docker.service

After following described steps you should be able to run docker engine and start / stop containers.

Lets now try to create a sample image. Please note that we do this on our host machine as our experience tells us that its always faster and does not require putting cups with cold water on top of Pi CPU. We search for “raspbian” on Docker hub and we find raspbian/stretch image which should be good to start with. We will add a boost library just to have some extra example layer.

FROM raspbian/stretch
RUN apt-get update && apt-get install -y \
   libboost1.62-all \
   && rm -rf /var/lib/apt/lists/*

We try to build it with:

$ docker build -t rpi_test .

And we get following error:

Sending build context to Docker daemon  2.048kB
 ...
 standard_init_linux.go:211: exec user process caused "exec format error"
 ...

So it is almost working. Just not quite yet. The problem is that our Pi is an arm image (which makes sense as Pi is an arm device). You can check it by running:

$ sudo docker image inspect raspbian/stretch | grep Arch
   "Architecture": "arm",

How to run an arm image on x86 architecture? Use emulator. Quemu was always helping us in such cases so lets make it do its magic for us.

$ sudo apt install qemu-user-static
$ docker run --rm --privileged multiarch/qemu-user-static --reset -p yes

First line installs emulator. The second tells our kernel to open arm (and other architecture) binaries with quemu. You can do this part manually by running. In this case we register only armv7 arch so you would need to repeat this step for every architecture you would like to use (with correct magic strings).

$ sudo sh -c "echo -1 > /proc/sys/fs/binfmt_misc/qemu-arm"
$ sudo sh -c "echo ':qemu-arm:M::\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-arm-static:OCF' > /proc/sys/fs/binfmt_misc/register"

Now we can not only build our custom images for Pi but also run them to inspect / adjust before deployment. You can push this image into your docker hub repo and try to pull it from the Raspberry Pi.

$ docker tag rpi_test <docker_id>/rpi_test
$ docker login
$ docker push <docker_id>/rpi_test

And on your Pi.

$ dokcer run -it <docker_id>/rpi_test /bin/bash

Should work :). 👍👍

* liking something twice results in not liking at all