Hello,
I never understand the way they take for n8n custom (but I learn on my own, so I probably do the wrong way 
)
But still it work enough for me
see my last post : Node:lts - no more the error npm run build - #6 by Dtneo
Dockerfile + docker-compose (+ add your own .env)
It work for me, I can write my own new node on my computer.
And for test, I just restart the docker-compose.
Good luck