CI: replace deprecated docker-image resource with oci-build-task

[neddp]

Replace the docker-image resource in the build-bpm-ci job with the canonical Concourse pattern: concourse/oci-build-task to build, then the existing bpm-ci-registry-image (registry-image type) to push.

The docker-image resource bundles a deprecated docker build toolchain that emits this warning on every run:

Login Succeeded
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            BuildKit is currently disabled; enable it by removing the DOCKER_BUILDKIT=0
            environment-variable.

The new pattern uses BuildKit, streams full stdout/stderr to the build log, and pushes via a Go-native registry client (no docker daemon).

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 12df73c and 081e0fa.

📒 Files selected for processing (1)

• ci/pipeline.yml

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Walkthrough

This PR refactors the CI pipeline to replace a local Docker build approach with OCI image task execution. The bpm-ci-registry-image resource is updated with an explicit tag: latest configuration. In the build-bpm-ci job, the dependency on bosh-golang-release-image is removed and the build step is rewired to use concourse/oci-build-task, which takes bpm-ci, runc-linux, and tini as inputs and builds from the Dockerfile. The resulting image artifact is then published directly to the registry instead of performing a local build/load cycle.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: replacing a deprecated docker-image resource with oci-build-task in the CI pipeline.
Description check	✅ Passed	The description is directly related to the changeset, explaining the motivation for replacing docker-image with oci-build-task and the benefits of the new approach.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/oci-build-task-for-build-dockerfile

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