Fix Docker build after build script addition

[haasonsaas]

Summary

• Copy build.rs into the Docker builder dependency-cache layer before the warm-up cargo build.

Why

The post-merge Publish image workflow for PR #101 failed because the Docker cache layer runs cargo build --release after copying only Cargo.toml, Cargo.lock, and migrations. Since PR #101 added build = "build.rs", Cargo expects build.rs during that warm-up build.

Validation

• git diff --check
• docker build --target builder -t diffscope-buildrs-hotfix:local . progressed past the previously failing dependency-cache layer; local Colima later OOMed during the final optimized musl link.
• Pre-push hook passed: workflow validation, Cargo version check, cargo fmt --check, cargo clippy --all-targets, npm ci, npm run build, npm test, and cargo test (1565 passed).