Ubuntu 22.04's protobuf-compiler (v3.12) lacks proto3 optional field
support. Install protoc v25.1 from GitHub releases instead, which
matches the protoc version available in Debian bookworm-based images
used by the other 5 services.
Validated: docker build completes successfully, binary starts with
ml_training_service --help.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>