Use non-root user in Dockerfile

Signed-off-by: ecaiyanlinux <martin.c.yan@est.tech>
Issue-ID: NONRTRIC-656
Change-Id: Ib85abd9de806b30b73af34a863b06c7663026c3d
diff --git a/test/mrstub/Dockerfile b/test/mrstub/Dockerfile
index 676c77c..a5f9ea0 100644
--- a/test/mrstub/Dockerfile
+++ b/test/mrstub/Dockerfile
@@ -34,4 +34,8 @@
 
 RUN chmod +x start.sh
 
+RUN groupadd -g 999 appuser && \
+    useradd -r -u 999 -g appuser appuser
+USER appuser
+
 CMD [ "./start.sh" ]
\ No newline at end of file