feat: bake product version from Spec and expose build in health.

Resolve MAJOR.MINOR via EventHubSpec/VERSION; CI run_number as EVENTHUB_BUILD; health returns version/build/git_sha/built_at.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker/Dockerfile

@@ -36,10 +36,12 @@ RUN collect-runtime-libs.sh /app/release /opt/runtime-libs
 FROM ${ERLANG_RUNTIME_IMAGE}
 
 ARG EVENTHUB_VERSION=0.0
+ARG EVENTHUB_BUILD=0
 ARG EVENTHUB_GIT_SHA=unknown
 ARG EVENTHUB_BUILT_AT=
 
 ENV EVENTHUB_VERSION=${EVENTHUB_VERSION} \
+    EVENTHUB_BUILD=${EVENTHUB_BUILD} \
     EVENTHUB_GIT_SHA=${EVENTHUB_GIT_SHA} \
     EVENTHUB_BUILT_AT=${EVENTHUB_BUILT_AT}