Switch to root user to delete build folder

Dockerfile

@@ -113,7 +113,9 @@ RUN cd MUST-v${MUST_VERSION}/build && \
     OMPI_MCA_rmaps_base_oversubscribe=true \
     make check-minimal -j8
 
+USER root
 RUN rm -r /build
 
 # Run container as non-privileged user
+USER user
 WORKDIR /home/user