[CI/Build][Intel] Add HPU image build with vllm-gaudi compatibility

[jakub-sochacki]

• Add HPU image build step that runs only on main branch
• Fetch compatible vLLM commit from vllm-gaudi's VLLM_STABLE_COMMIT
• Use dynamic image tagging based on stable commit, not BUILDKITE_COMMIT
• HPU images always pushed to vllm-ci-postmerge-repo registry
• Don't use static docker_image_hpu variable (it's determined dynamically)
• Implement image existence check before building to avoid duplicates
• Build HPU images with both VLLM_COMMIT and VLLM_GAUDI_COMMIT args

This enables nightly HPU benchmarks by ensuring Docker images are built with vLLM versions compatible with the vllm-gaudi plugin, addressing version synchronization issues between the two repositories.