perf: cache conformance image builds

[lucarlig]

Summary

• persist the Linux Rust target/ directory across source-mode Docker builds
• keep the built binary outside the cache mount for the final runtime image
• exclude Git and integration runtime state from the Docker context
• pin conformance CI to cf-integration 0.2.1

Normal Linux CI still uses the prebuilt artifact path. The cache accelerates local/source builds without trying to copy an incompatible macOS target/ binary into Linux.

Results

• warm Docker image build: 0.90s
• external-dataplane make conformance: 136.35s, down from 364.88s
• the strict-modern routed registration failure is still printed as a failed scenario and returns nonzero; completed fixture and client results are preserved

Verification

• actionlint
• docker buildx build --check --build-context prebuilt=target/debug --file docker/Dockerfile .
• runtime image --help smoke test
• make -n conformance
• make -n conformance-bless
• end-to-end make conformance with the locally built 0.2.1 CLI

Depends on contextforge-org/contextforge-dev-tools#12. The remaining routed failure is the control-plane strict-modern resource-sync issue tracked in IBM/mcp-context-forge#6485.

[lucarlig]

/conformance

[cafalchio]

LGTM