set pgport if !5432 on docker

docker has [a script](https://github.com/geopython/GeoHealthCheck/blob/a25e87125eec4eea57ab26999d67f89dbf5f0872/docker/scripts/configure.sh#L55) that checks if postgres is running prior to starting the webapp
this script assumes the default port, usually 5432
but you can set a [environment variable PGPORT](https://www.postgresql.org/docs/current/libpq-envars.html) to the relevant db port/. Psql will then use that port

typically you will get this type of messages, if there is a bad port

```
Check if Postgres is avail/ready...
12 example.com:5432 - no response
11 Exit code=2 - Postgres not ready - sleeping
10 example.com:5432 - no response
```

suggestion to include this aspect in docker documentation