Navigating Docker for Windows versions

Recently, gitlab-runner got support for running jobs inside windows containers. This simplifies a lot the setup needed to get a windows CI job up and running, and it makes it similar how we do linux builds.

Windows though has a couple of gotchas, the behavior of docker on windows can vastly vary depending on which binary and/or configuration you use.

Containers on windows are dependent on the server version of the Host. For example, your server 2016 (1607) containers can only be executed on a server 2016 host. Currently there are 2 popular base versions that docker supports, Server 2016, and 2019. Gitlab-runner only supports server 2019, so we will go with that.

Microsoft recently introduced a set of  Process isolation apis, where up till now docker had been using a hyper-v backend to launch containers. This results in possibly more light-weight containers as opposed to light VMs with Hyper-V, but also now now docker for windows has 2 isolation backends, which can affect differently the behavior of your containers! Proccess Isolation seems to be the default for Server 2019.

Next you want to go and install docker, so you look up the Microsoft documentation. There’s one option for the servers, but there’s also an option for Docker Desktop for Windows 10, and to add to this Docker Desktop itself has 2 versions depending if you want the Community or Enterprise Edition! Also, you can’t install Docker on windows 10 the same way you would do on the Server, so you are left having to mix different binaries and hope everything works out. Spoiler, it didn’t work for me! There were couple of build/networking failure when using Docker Desktop, where the same Dockerfile built fine on the server host.

Bonus round:

Docker Desktop has 2 modes, one for running linux containers and one for windows containers! We talked about the 2 different isolation backends for windows above. Now it looks like the linux containers mode got a backend based on WSL2 in addition to the existing hyper-v backend it has been using.

Hope you made it to the end without losing count of all the different “containers” in the windows land. :)