Updated README.md documentation. Included note on docker limitations.

contrib/docker/README.md

@@ -6,6 +6,22 @@ From this directory you can:
 
 will build Docker image for building NGraph-Cpp and running unit tests
 
+### make check_cpu
+
+will set up a cmake BUILD directory and run "make check" (unit tests) in
+the docker image
+
 ### make shell
 
 will put you into a bash shell with NGraph-Cpp installed
+
+### Notes
+
+Due to limitations in how docker mounts work, "make check_cpu" will fail
+if you try to run it from an NFS-mounted directory.  The cause results
+from the process in the docker container running as root.  When a file or
+directory is created by root in the mounted directory tree, from within
+the container, the NFS-mount (in the host OS) does not allow a root-created
+file.  This is dependent on whether the host OS performs "root squash" when
+mounting NFS filesystems.  The fix to this is easy: run "make check_cpu" from
+a local filesystem.