Mount --user-data-dir for container

[phit42]

I was trying to follow this suggestion (for my docker container install of sockpupp on a Synology NAS):

Don't burn out your disk!! Mount the path for --user-data-dir as a RAM Disk/tmpfs disk ! This will also help to speed up Chrome

so I mounted a directory /volumeSATA/satashare/.../user-data-dir from an external sata drive (I cannot "ram" it but at least I can move it away from my raid drives),
-v /volumeSATA/satashare/.../user-data-dir:/user-data-dir:rw
and told Chromium to use it (env CHROMIUM_FLAGS --user-data-dir=/user-data-dir ).

It doesn't work though. it doesn't recognize the flag or the directory, what am I missing?