Put a container user into a group, how to?

I have been using docker for a very short time.
I found portainer.io very practical and easy to use.
Now I want to go further, I shared a USB port from my Pi, it worked, it is well in the container.
But, the user under which the container is running does not have the rights to use the ttyUSB0 port.
If I understand correctly I have to add this user to the dialout group.

In Docker command line mode it looks like this:
USER root
RUN addgroup node-red dialout
USER node-red

Now how to give this information to Portainer.io ?
Thanks for help.

What i see inside the container