fix(ci): align docker-buildx with zookeeper-operator baseline for multi-arch signing

[whg517]

Problem

The Release Image job's Sign operator image step failed with exit code 2 during the 0.4.0-dev tag push release workflow (run #27288833153). All other 11 jobs passed successfully.

Failure chain:

Release Image
  ├─ Build and push operator → ✅ success
  └─ Sign operator image     → ❌ exit code 2

Root Cause

docker-buildx was missing the Dockerfile.cross generation step that injects --platform=${BUILDPLATFORM} into the builder stage FROM line. Without this, buildx multi-arch builds may produce an incomplete or malformed docker-digests.json metadata file, causing the sign-image composite action's jq parse to fail.

Changes

• Add sed command to generate Dockerfile.cross with --platform=${BUILDPLATFORM}
• Use Dockerfile.cross instead of Dockerfile for the buildx build
• Hardcode docker-digests.json as metadata file (matching zookeeper-operator)
• Remove unused BUILDX_METADATA_FILE variable
• Clean up Dockerfile.cross after build

Verification

- Aligns with zookeeper-operator baseline (the reference 0.4.0-dev release)

Diff against zookeeper-operator docker-buildx target: zero difference.