How can I store a Docker build artifact from a multi-stage build in Cloud Build?

When Cloud Build collects artifacts (using the artifacts section in your cloudbuild.yaml), it looks for files in the build’s workspace—not inside the filesystem of your container during the Docker build process. In your example, the file /app/test.log is created inside the container during the Docker build, so it isn’t available in the workspace for Cloud Build to upload.

There are a couple of approaches to solve this:

1. Copy the Artifact to the Workspace

Add an extra build step (or modify your Dockerfile) so that the generated artifact is copied from the container to the Cloud Build workspace. For example, you can create a final step in your Dockerfile that copies the file to a known location under the workspace (which is mounted at /workspace in all build steps):

DOCKERFILECopied
# Use a new stage that has access to the artifact and the workspace
FROM alpine AS export-artifact
WORKDIR /workspace
# Copy the test.log from the build stage
COPY --from=build /app/test.log .

Then, in your cloudbuild.yaml, build this final image (or run a container from it) and ensure that the file appears in the workspace. For example:

YAMLCopied
steps:
  - name: 'gcr.io/cloud-builders/docker'
    args: ['build', '-t', 'myapp:latest', '.']
  - name: 'gcr.io/cloud-builders/docker'
    args: ['build', '--target', 'export-artifact', '-t', 'export-artifact:latest', '.']
  - name: 'gcr.io/cloud-builders/docker'
    args: ['run', '--rm', 'export-artifact:latest']

Because the final stage copies the file to /workspace, Cloud Build will see it. Then you can refer to it in the artifacts section like so:

YAMLCopied
artifacts:
  objects:
    location: 'gs://mybucket/test-results'
    paths: ['test.log']

2. Use a Separate Step to Extract the Artifact

Another option is to run a container from your built image and then copy the file out into the workspace. For example:

YAMLCopied
steps:
  - name: 'gcr.io/cloud-builders/docker'
    args: ['build', '-t', 'myapp:latest', '.']
  - name: 'gcr.io/cloud-builders/docker'
    entrypoint: 'bash'
    args:
      - '-c'
      - |
        CONTAINER_ID=$(docker create myapp:latest)
        docker cp $CONTAINER_ID:/app/test.log ./test.log
        docker rm $CONTAINER_ID

Then, in your artifacts section, reference the file relative to the workspace:

YAMLCopied
artifacts:
  objects:
    location: 'gs://mybucket/test-results'
    paths: ['test.log']

Summary

Remember:

• Cloud Build’s artifact collector only sees files in the workspace.
• Files created solely inside a Docker container during a build are not automatically available in your build workspace.
• Use an extra step (or modify your multi-stage Dockerfile) to copy the artifact (in this case, /app/test.log) to a path in the workspace (for example, ./test.log).

This way, Cloud Build will be able to find the file and upload it to your Cloud Storage bucket without any errors.