Where is the root directory of node-red in the docker image? – General

I feel this is my case as well. Could you please explain that using words for neewbie? I know that using Docker some user data is kept inside container folder and this one in mapped to the external (external to the container) folder /node-red/data right? So if i want to install new packages i need to run
nmp install command form /data folder – so not from inside of the container but using Pi terminal, right?

Yes, i was reading the above mentioned docs, however, Linux, Docker is still kinda magic for me, sorry.