Fix Dockerfile.sdk export stage inheriting build artifacts.

The export stage must continue from workload-build; starting a fresh Maven
image dropped /src and broke copying the packaged workload jar.

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

Author: KirillKurdyukov

slo-workload/docker/Dockerfile.sdk

@@ -33,7 +33,7 @@ RUN SDK_VERSION="$(cat /tmp/sdk.version)" && \
         -Dmaven.javadoc.skip=true \
         package
 
-FROM ${MAVEN_IMAGE} AS export
+FROM workload-build AS export
 ARG WORKLOAD_MODULE
 ARG WORKLOAD_JAR
 ARG COPY_LIBS