Add update statement to manylinux Dockefile instructions. (#329)

Author: mc-nv

tools/gen_ort_dockerfile.py

@@ -142,7 +142,8 @@ def dockerfile_for_linux(output_file):
 ENV PYTHON_BIN_PATH=${PYBIN}/python${PYVER} \
     PATH=${PYBIN}:${PATH}
 
-RUN dnf install -y \\
+RUN dnf update && \\
+    dnf install -y \\
         ca-certificates \\
         curl \\
         git \\