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3.BUILD_APP.md

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3. Build the application as a container image

To build the app as a container image we will create a simple Dockerfile.

It consists of a multistage build with two stages:

  • Stage 1: Install dependencies and build the app from a Golang image
  • Stage 2: Copy the final binary into the runtime image

You can build the image as follows:

docker build . -t k8s-admission-controller:latest

Test the image is running fine

If you try to run a container from this image without any argument it will try to load certificates from the default location. Since there is none in the image execution will fail:

docker run -it k8s-admission-controller

E0325 22:53:33.427023       1 main.go:32] Filed to load key pair: open /etc/certs/app.crt: no such file or directory
E0325 22:53:33.427023       1 main.go:32] glog: exiting because of error: log: cannot create log: open /tmp/k8s-admission-controller.2fbeb4dcab82.webhook.log.INFO.20230325-225333.1: no such file or directory

You can pass the local directory ($PWD) as a volume and mount it in the /etc/certs folder. Ex:

docker run -it -v $PWD/certs:/etc/certs k8s-admission-controller -logtostderr

Once you make sure the image is working as expected you can push it to your Container Registry (CR). In my case, that's ghcr.io.

Retag the image to match your CR:

 docker tag k8s-admission-controller ghcr.io/felipempda/k8s-admission-controller

Login to the CR with your user and Personal Access Token.

docker login

And push the image:

docker push ghcr.io/felipempda/k8s-admission-controller

You can also set up a workflow to that automatically for you.