Skaffold does not cope well with buildkit used with --output (invalid reference format)

[ifalex]

Expected behavior

When using buildkit in conjunction with --output skaffold should not expect that docker will tag the build.

Actual behavior

Docker buildkit used with --output does not produce any tag for the build. Skaffold on the other hand is not aware about this behavior and expects a tag failing the process.

Error parsing reference: "" is not a valid repository/tag: invalid reference format

Information

• Skaffold version:

v1.35.2

• Operating system:
  Running from a docker container based on:
  CentOS Linux release 7.9.2009 (Core) (Linux 6e75c71d7fd8 4.18.0-348.7.1.el8_5.x86_64
• Installed via:
  skaffold.dev
• Contents of skaffold.yaml:

---
apiVersion: skaffold/v2beta26
kind: Config
build:
  local:
    push: false
    tryImportMissing: true
    useBuildkit: true
    useDockerCLI: true
  tagPolicy:
    envTemplate:
      template: '{{.GIT_COMMIT}}'
  artifacts:
  - image: my-docker-image
    context: source/my-docker-image
    docker:
      buildArgs:
        GIT_COMMIT: 75c25fd
        GIT_BRANCH: <branch_name>
      noCache: false
      target: export-test-results
      cliFlags:
      - --output
      - /tmp/output_dir

Steps to reproduce the behavior

1. Using a multistage docker build with a target and running skaffold on that target to get the output files (unit tests at build time)
   https://docs.docker.com/engine/reference/commandline/build/#custom-build-outputs

FROM scratch as export-test-results
COPY --from=build /my-app/build/reports/jacoco/* /

From skaffold perspective this will resume to following skaffold build execution:

2. skaffold build -f /tmp/skaffold-my-image-export-test-results.yaml --quiet=false --cache-artifacts=false --default-repo=my-docker.repository
   Which will fail with:

Error parsing reference: "" is not a valid repository/tag: invalid reference format

By using vdebug I got the command that was executed de-facto by skaffold.

DOCKER_BUILDKIT=1 docker build source/my-docker-image --file /my_image_dir/source/my-docker-image/Dockerfile -t my-docker-image:1.0.0 --build-arg GIT_BRANCH=feature/random-branch-name --build-arg GIT_COMMIT=0895bd5 --output /tmp/my_custom_dir3 --target export-test-results --force-rm

Basically what happens in behind is when using DOCKER_BUILDKIT=1 with --output the normal behaviour is not to tag the image.

Skaffold doesn't take this in consideration and expects a tag, failing with the error message from above which in this case isn't the correct behavior.

[ifalex]

This can be mitigated by setting "useBuildkit": False or by creating two different skaffold files and execute them in order.
One without output and then the other with --output.
If we set buildkit on False then we lose the benefits that buildkit introduces so wouldn't be the best decision.
The second option is a bit ugly because you wouldn't like to do this in 2 steps.