LGTM — no issues found.

The SMT implementation is clean: stripObjectKey correctly handles V1 master, V1 version, and V0 legacy encodings; extractDocumentKeyId navigates the Struct defensively with appropriate debug logging for mismatches; the partition is correctly set to null so Connect re-hashes on the stripped key. Tests cover all encoding variants plus edge cases (empty keys, missing fields, non-string IDs). The Dockerfile build stage wiring is correct — kafka_version flows through kafka_build_vars.sh → env.kafka_version → build-arg KAFKA_VERSION → -Dkafka.version=, ensuring the SMT compiles against the shipped Kafka release. The prior review feedback (tests skipped via -DskipTests) has been addressed.