Speed up local MCP conformance builds

[lucarlig]

Summary

• add a dedicated, portable conformance Dockerfile that builds only the data-plane binary
• persist Cargo registry, git, and target caches across local conformance builds
• keep Redis, nginx, and the control plane on their published images
• make conformance the only Make entry point and remove the redundant conformance-image target
• leave Cargo profiles and the production Dockerfile unchanged; CI invokes the production Dockerfile directly

Measurements

Measured locally with Docker BuildKit:

• cold image build: 3m49s → 1m40s
• rebuild after a library source edit: 15.6s
• warm full make conformance: 43.1s, including fixture setup and all 50 scenarios

Compilation stays inside Docker for host portability. The conformance image uses the dev profile because this is a correctness workflow, while production CI continues to validate the release image.

Validation

• MCP_CONFORMANCE_PORT=18080 make conformance (expected-failure baseline passed)
• docker buildx build --check --platform linux/amd64,linux/arm64 -f docker/conformance.Dockerfile .
• actionlint .github/workflows/mcp_conformance.yml
• uvx pre-commit run --all-files