Container port unreachable from host though mapped/exposed

Thank’s for your answer, but this has nothing to do with our problem.

Maybe I wasn’t clear on what I want to achieve. I have two containers: one with the app and another one with the db. I want to reach the app through port 3000. Therefor I mapped the container’s port 3000 to the host’s port 3000 with

ports:
  - '3000:3000'

But the app’s container cannot be reached from the host via port 3000.

I can reproduce this on several PCs.

So the question is: How can I make the app, listening on port 3000, accessible from my host machine? What am I missing?