"networks:" has to be on the same level as "services:"
That was it. I had it indented with volumes and ports. But I like the shorter solution better, which IS indented with volumes and ports.
Did you check on the host as well?
Or just try another port like 3001
I just looked down the container list in Portainer for Published Ports. Not seeing 3000 in the list I assumed I had done something wrong. Trying the port in my browser Digikam popped up, which had no port published in Poretainer. Changing the port to - 3000:3001 did the trick.
Now on to the reverse proxy.