Creating Windows Container Images Without Docker • Jamie Phillips

Crane is a tool created by Google for working with images and registries. Crane has a lot of excellent functionality, and the one that is the most interesting is the append command. With the append command, we can add a layer to a container image. This feature means that we can create a layer with our application and append it to an existing base image. What this does is allow us to create a container regardless of the OS or requiring Docker. Let’s see how that is accomplished by adding a .NET App to the ASP .NET Runtime Nano server 2022 image on a Linux workstation. You can install Crane as a binary or run:

go install github.com/google/go-containerregistry/cmd/crane@latest

Example .NET App

First, we need to create a new ASP .NET application using the dotnet CLI.

dotnet new mvc --output craneapp

Now let’s publish the application to generate our compiled application.

cd craneapp/
dotnet publish -c release -o ./app

We have a project published to the app directory that we can bundle for adding to the base image.

Creating the bundle

The bundle required for the append command is a tar file. On Linux, that could be a tar of the output of the publish command. However, on Windows, the layers are required to have a specific structure. Windows Layers are required to have a Files and a Hives directory. Inside of the Files directory is where you would place your published output. Then those directories need to be bundled together in a tar file. Fortunately, the lastest crane handles that complexity for us so we can just tar up our app directory.

# creating the tar file
tar -cf layer.tar app/

We should have a layer.tar file in our that we can use when we run the crane command to create our image.

Appending

The Crane append command, by default, wants to push to a registry. I am using Docker Hub, so log into your registry first before executing the append command. Let’s run the append making sure to set our base image, mcr.microsoft.com/dotnet/aspnet:6.0-nanoserver-ltsc2019.

crane append /
--platform=windows/amd64 /
-f layer.tar /
-t phillipsj/craneapp:0.1.0 /
-b mcr.microsoft.com/dotnet/aspnet:6.0-nanoserver-ltsc2019

Once that finishes, we should have an image in Docker Hub. However, we are missing our entry point; fortunately, crane doesn’t currently support mutating the entry point for Windows images. That means we will have to pass an entry point when executing the command which Kubernetes and Docker support.

Executing on Windows

Okay, I have switched to my Windows box with Docker Desktop running Windows containers. Let’s pull the image and run it.

docker run -it -p 5000:80 phillipsj/craneapp:0.1.0 dotnet craneapp.dll

Now we can navigate to the app in our browser to make sure it works. You should see the default app at http://localhost:5000.

Wrapping Up

This capability is way cool and allows the creation of Windows container images without needing Docker Desktop or a Dockerfile. The append command does require that you are using something that doesn’t require compilation in a container. Go, .NET, Java, JS, Python, and others will work just fine as long as you choose the appropriate base image. This command will not work if you need to leverage IIS and do a feature installation as that requires modifying the base Windows OS, not just adding your application files. Append also opens up a lot of other scenarios, like being able to build your Windows containers on Linux CI servers.

Thanks for reading,

Jamie

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