ci(release-server): wire openapi build context for dashboard stage

[dvcdsys]

Summary

Both server/Dockerfile and server/Dockerfile.cuda have COPY --from=openapi openapi.yaml /spec/openapi.yaml in their dashboard build stage (added when the dashboard started consuming the OpenAPI spec for type generation). The release workflow's docker/build-push-action calls were missing the corresponding build-contexts: openapi=doc parameter, so the next `server/v*` tag push (or workflow_dispatch) would fail at the dashboard stage with stage openapi not found.

make docker-build-cuda-dev works because it passes --build-context openapi=$(ROOT)/../doc directly. This brings CI to parity.

Must merge BEFORE `server/v0.5.0` tag — otherwise the release CI run will fail.

Verification

Verified end-to-end via workflow_dispatch test run #25315845271:

Job	Result
Build CPU image (multi-arch)	✅ success
Build CUDA image (amd64)	✅ success
Publish release	skipped (correctly — gated to `event_name == 'push'`)

Pushed test images to Docker Hub at `dvcdsys/code-index:ci-test-openapi` (CPU) and `:ci-test-openapi-cu128` — production tags `:latest` / `:cu128` were not touched (`update_floating_tags=false`).

Test plan

• CPU job builds and pushes successfully
• CUDA job builds and pushes successfully
• Production floating tags untouched
• Reviewer can delete `:ci-test-openapi*` tags from Docker Hub at leisure

🤖 Generated with Claude Code

[dvcdsys]

Superseded by #28 — the workflow fix logically belongs with the dashboard work that introduced the dependency on openapi=doc build context (the COPY --from=openapi line in Dockerfile.cuda was added when the dashboard build stage landed). Cherry-picked as commit c9d0ceb on dashboard and verified end-to-end via the earlier workflow_dispatch test run #25315845271. Closing this branch — single PR is cleaner for release ordering.