How can I update an Image in Google Artifact Registry?

Question

When viewing Images in Google Cloud Platform's Artifact Registry, there is an "Updated" time column. However, whenever I build the same image and push it again, it creates a new image.

As part of a Cloud Build process, I am pulling this Ruby-based image, updating gems, then pushing it back to the Artifact Registry for use in later build steps (DB migration, unit tests). My hope is that upon updating the Ruby gems, nothing would happen in most cases, resulting in an identical Docker Image. In such a case, I'd expect no new layers to be pushed. However, every time I build, there is always a new layer pushed, and therefore a new Artifact.

Thus, the problem may be with how Cloud Build's gcr.io/cloud-builders/gsutil works rather than Artifact Registry itself. Here're my relevant build steps in case it matters:

  - id: update_gems
    name: 'gcr.io/cloud-builders/docker'
    args: [ 'build', '-t', 'us-central1-docker.pkg.dev/$PROJECT_ID/{my repo}/{my image}:deploy',
            '-f', 'docker/bundled.Dockerfile', '.' ]
  - id: update_image
    name: 'gcr.io/cloud-builders/docker'
    args: [ 'push',  'us-central1-docker.pkg.dev/$PROJECT_ID/{my repo}/{my image}:deploy' ]

The first step refers to "bundled.Dockerfile" which has these contents:

FROM us-central1-docker.pkg.dev/{same project as above}/{my repo}/{my image}:deploy
WORKDIR /workspace
RUN bundle update
RUN bundle install

Is there a way to accomplish what I'm currently doing (ie update a Deploy-time container used to run rspec tests and run rake db:migrate without making new images every time we build? I assume those images are taking up space and I'm getting billed for it. I assume there's a way to "Update" an existing Image in the Artifact Registry since there is an "Updated" column.

Answer 1

You are not looking at container "images". You are looking at "layers" of an image. The combination of layers results in a container image. These can also be artifacts for Cloud Build, etc.

You cannot directly modify a layer in Artifact Registry. Any changes you make to the image creation will result in one or more layers changing while results in one or more new layers being created. Creating an image does usually does not result in all layers changing. Your new image is probably the result of old and new layers. Layers are cached in Artifact Registry for future images/builds.

More than one container image can use the same layers. If Google allowed you to modify individual layers, you would break/corrupt the resulting containers.