How can I export Docker build artifacts from a container in Cloud Build?

When you try to specify a path like /app/test.log in your artifacts section, Cloud Build can’t find that file because it’s inside your Docker container rather than in the build workspace. Cloud Build’s artifact mechanism only collects files from the workspace (the filesystem outside of your container).

To fix this, you have two typical options:

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Option 1: Copy the File to the Workspace

After your Docker image is built, add a build step that runs a container from the image and copies the file (using a command like docker cp) into your workspace. For example:

YAMLCopied
steps:
  # Build the Docker image (using multi-stage build)
  - name: 'gcr.io/cloud-builders/docker'
    args: ['build', '-t', 'my-image', '.']

  # Run a container and copy the artifact out to the /workspace directory
  - name: 'gcr.io/cloud-builders/docker'
    entrypoint: 'bash'
    args:
      - '-c'
      - |
        container=$(docker create my-image)
        docker cp ${container}:/app/test.log ./test.log
        docker rm ${container}
  
artifacts:
  objects:
    location: 'gs://mybucket/test-results'
    paths: ['test.log']

In this example, the second step creates a container from your image, copies /app/test.log into the build workspace as test.log, and then removes the container. The artifact is then found in the workspace and uploaded to your Cloud Storage bucket.

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Option 2: Modify the Dockerfile to Emit Artifacts into a Mounted Workspace Directory

Another approach is to have your Dockerfile write its outputs directly into the workspace directory. You can do this by mounting the workspace into the container when running the build portion that generates the artifact. For example:

1. Change your Cloud Build step to run the container with the workspace mounted:

   YAMLCopied
   steps:
     - name: 'gcr.io/cloud-builders/docker'
       args: ['build', '-t', 'my-image', '.']
   
     - name: 'gcr.io/cloud-builders/docker'
       entrypoint: 'sh'
       args:
         - '-c'
         - |
           docker run --rm -v "$$PWD:/workspace" my-image sh -c "cp /app/test.log /workspace/test.log"
   
   artifacts:
     objects:
       location: 'gs://mybucket/test-results'
       paths: ['test.log']

2. Or adjust your Dockerfile so that it writes to a persistent location mounted as /workspace.
   For example, if you add a copy command in the Dockerfile to copy the log to a known directory that is later exported, then your artifact step would refer to that file.

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Summary

Cloud Build’s artifacts are collected from the build’s workspace. Files generated inside a Docker container aren’t directly visible to Cloud Build. You must explicitly copy those files out into the workspace to upload them as artifacts.

Choose the method that fits your workflow best.