restore multi-arch Docker publish
1.1.0 published no GHCR image — the build stage was target-platform,
so Bun 1.4.0 aborted under QEMU x86_64 emulation and the multi-arch
buildx push failed outright. npm, the MCP Registry, and the GitHub
Release all shipped fine for 1.1.0; only Docker was missing.
- Docker build stage pinned to $BUILDPLATFORM (cyanheads/mcp-ts-core#370)