When deploying steps/02-docker, fly-proxy can't reach app at port 3000

[kanian]

$  fly  deploy
WARNING The app is not listening on the expected address and will not be reachable by fly-proxy.
You can fix this by configuring your app to listen on the following addresses:
  - 0.0.0.0:3000

Solution:
In steps/02-docker/Dockerfile, add:

# set environment variables
...
ENV SERVER_PORT="3000"

In steps/02-docker/app/index.ts, change:

.listen(process.env.PORT...

to

.listen(process.env.SERVER_PORT...