reapply docker build optimizations to Dockerfile.source

[dazthecorgi]

Commit 1bdc940 ("current state of v2.1.0.24") accidentally reverted the build optimizations introduced in PR #551 while landing unrelated changes (ubuntu:22.04 base, upstream protoc, the build-migrate-tool RocksDB stage, qns-api removal). This restores the optimizations on top of those changes:

• Consolidate the seven parallel gen-* stages (each running generate.sh) into a single gen-rust stage: one cargo build for all leaf crates so shared workspace deps compile once instead of seven times. Artifacts are stashed in /out/ since target/ is a cache mount, and build-context now COPYs --from=gen-rust accordingly.
• Restore cargo cache mounts (cargo-registry, cargo-git, cargo-target-node) on build-node.
• Restore Go cache mounts (go-build, go-mod) on build-qclient so a build-context invalidation no longer forces a from-scratch Go rebuild.
• Extend the same Go cache mounts to the new build-migrate-tool stage for consistency.
• Restore the explanatory comments on COPY crates and build-qclient.

This brings Dockerfile.source back in line with Dockerfile.sourceavx512, which was untouched by the revert and shares the cargo/go cache ids.