Push Nix-built container image to GCR using github action

Post on February 7, 2022
Updated on Feb 07, 2022
nix, github, skopeo


I have been trying out google cloud run with a simple haskell + purescript web app. In order to deploy the app, I need to containerize the app and push the docker image to google container registry. I wanted to figure out whether this can be done with nix and some simple github actions. Turns out: with skopep, it is pretty easy!

skopeo copy

After some searching, I found this discourse thread mentions skopeo, and I found this repo using nix flakes and skopeo to push docker image into a private docker registry in github action. So I decided to try out skopeo.

skopeo can push an image from one location to another. It supports Google Container Registry (GCR)

So we can push our image like this

skopeo copy docker-image://$(nix-build -A my-image --no-out-link) docker://gcr.io/google-cloud-project-name/service:tag

install skopeo in github action

I am using install-nix-action in my build step already, so I added a shell script bin in my nix flake, and make it as nix flake app using flakes-utils, which can be run using nix run ".#script"

upload-script = pkgs.writeShellScriptBin "upload-image" ''
    set -eu
    OCI_ARCHIVE=$(nix-build --no-out-link)
    ${pkgs.skopeo}/bin/skopeo copy --dest-creds="_json_key:$GCR_DEVOPS_SERVICE_ACCOUNT_KEY"     "docker-archive:$OCI_ARCHIVE" "$DOCKER_REPOSITORY"
apps.upload-script = flake-utils.lib.mkApp { drv = upload-script; };

Authenticate skopeo with GCR

I was not able to find too much documentation on this step. So I went to some trail and error.

GCR supports different authentication methods. I thought the easiest would to create a service account key with Cloud Storage Admin role, and upload the key content as a secret in github actions.

docker login -u _json_key -p "$(cat keyfile.json)" https://HOSTNAME

Of course, we want to replace docker with skopeo. This works locally, but not in github action due to how skope login works.

But turns out, we don’t have to do skope login, we can just using skopeo copy --dest-creds.

Put everything together

So I put my service account key, google cloud project name, and the name of the GCR docker image name into github actions as secret, and pass them as env variable in the step, and do nix run ".#upload-script"

name: CI

on: [push, pull_request]

    name: Build
    runs-on: ubuntu-latest
    - uses: actions/checkout@v2.4.0
    - uses: cachix/install-nix-action@v16
        extra_nix_config: |
          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
    - name: Push Image
      run: |
        nix run ".#upload-script"
      if: github.ref == 'refs/heads/master'

Works like a charm. Thanks for reading


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