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

```
$ 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...
```