Adventures in Task Cluster – running a custom Docker image

I needed to get compiz on the machine (bug 1223123), and I thought it should be on the base image.  So to take the path of most resistance, I dove deep into the internals of taskcluster/docker and figured this out.  To be fair, :ahal gave me a lot of pointers, in fact if I would have taken better notes this might have been a single pass to success vs. 3 attempts.

First let me explain a bit about how the docker image is defined and how it is referenced/built up.  We define the image to use in-tree.  In this case we are using taskcluster/docker-test:0.4.4 for the automation jobs.  If you look carefully at the definition in-tree, we have a Dockerfile which outlines who we inherit the base image from:

FROM          taskcluster/ubuntu1204-test-upd:

This means there is another image called ubuntu1204-test-upd, and this is also defined in tree which then references a 3rd image, ubuntu1204-test.  These all build upon each other creating the final image we use for testing.  If you look in each of the directories, there is a REGISTRY and a VERSION file, these are used to determine the image to pull, so in the case of wanting:

docker pull taskcluster/desktop-test:0.4.4

we would effectively be using:

docker pull {REGISTRY}/desktop-test:{VERSION}

For our use case, taskcluster/desktop-test is defined on  This means that you could create a new version of the container ‘desktop-test’ and use that while pushing to try.  In fact that is all that is needed.

First lets talk about how to create an image.  I found that I needed to create both a desktop-test and an ubuntu1204-test image on Docker Hub.  Luckily in tree there is a script which will take a currently running container and make a convenient package ready to upload, some steps would be:

  • docker pull taskcluster/desktop-test:0.4.4
  • docker run taskcluster/desktop-test:0.4.4
  • apt-get install compiz; # inside of docker, make modifications
  • # now on the host we prepare the new image (using elvis314 as the docker hub account)
  • echo elvis314 > testing/docker/docker-test/REGISTRY
  • echo 0.4.5 > testing/docker/docker-test/VERSION  # NOTE: I incremented the version
  • cd testing/docker
  • docker-test # go run a 5K
  • docker push elvis314/docker-test # go run a 10K

those are the simple steps to update an image, what we want to do is actually verify this image has what we need.  While I am not an expert in docker, I do like to keep my containers under control, so I do a |docker ps -a| and then |docker rm <cid>| for any containers that are old.  Now to verify I do this:

  • docker pull elvis314/desktop-test:0.4.5
  • docker run elvis314/desktop-test:0.4.5
  • compiz # this verifies my change is there, I should get an error about display not found!

I will continue on here assuming things are working.  As you saw earlier, I had modifed filed in testing/docker/desktop-test, these should be part of a patch to push to try.  In fact that is all the magic.  to actually use compiz successfully, I needed to add this to to launch |compiz &| after initializing Xvfb.

Now when you push to try with your patch, any tasks that used taskcluster/desktop-test before will use the new image (i.e. elvis314/desktop-test).  In this case I was able to see the test cases that opened dialogs and windows pass on try!


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