[CI] Remove the unused ECR cache lookup from pipeline generation

[Hoiyeuhng]

Summary

• stop injecting $CACHE_FROM and $CACHE_TO into step commands
• remove get_ecr_cache_registry() and the two helpers only it used
• add regression coverage that neither placeholder is injected

Net +9 / -97. Registry caching behavior is unchanged.

Why

_get_variables_to_inject() resolves the two placeholders through get_ecr_cache_registry(), which runs aws ecr get-login-password, docker login and up to two docker manifest inspect calls, then raises RuntimeError if the login fails.

Nothing consumes the result. In vLLM main's .buildkite/ tree the placeholders occur only inside image_build/image_build.sh, image_build/image_build_cpu.sh and scripts/ci-bake-rocm.sh, each of which computes its own values just before the build — true since vllm-project/vllm#29343 moved image builds under .buildkite/image_build/. Release branches v0.28.0, v0.27.0 and v0.9.2 have no occurrences either.

The other injected variables are still live, which is what isolates these two. image_build.yaml consumes five of the seven as positional arguments:

.buildkite/image_build/image_build.sh $REGISTRY $REPO $BUILDKITE_COMMIT $BRANCH $IMAGE_TAG $IMAGE_TAG_LATEST

So the cost today is less the round trip than that _get_variables_to_inject() reads as though the generator still owns cache-tag resolution.

Scope

bootstrap-intel.sh has its own bash resolve_ecr_cache_vars that also looks unused, but it sits on a live bootstrap path whose caller is configured in Buildkite rather than in this repo, so I left it alone. Happy to follow up if you confirm it.

Duplicate-work check

#394 changes ci.hcl and terraform rather than the generator, and #413 is explicitly foundation-only, so neither overlaps. #321 edits the same function, so whichever lands second needs a trivial conflict fix.

Validation

• pytest buildkite/tests — 111 passed on Python 3.12 and on 3.9 (the pipeline agents run 3.9), installed the way the Pipeline generator tests workflow does.
• The new test fails against current main before passing here.
• pre-commit clean on the changed files except ruff-format, which reformats buildkite_step.py and test_step.py identically at main — pre-existing, so left out of this diff.

AI assistance

AI assistance was used to research, implement and test this change. The submitter has reviewed every changed line.