added colon in dockerfile

Author: pradeepjha-microsoft

App/kernel-memory/Dockerfile

@@ -37,7 +37,7 @@ RUN \
     #Debian: useradd --create-home --user-group $USER --shell /bin/bash && \
     adduser -D -h /app -s /bin/sh $USER && \
     # Allow user to access the build
-    chown -R $USER.$USER /app
+    chown -R $USER:$USER /app
 
 COPY --from=build --chown=km:km --chmod=0550 /app/publish .