kong initialize postgres error

Hi there：

I've tried to connect Kong to external DB Postgre.
The scenario:
```
Error:
/usr/local/share/lua/5.1/kong/cmd/utils/inject_confs.lua:28:
stack traceback:
[C]: in function 'assert'
/usr/local/share/lua/5.1/kong/cmd/utils/inject_confs.lua:28: in function 'load_conf'
/usr/local/share/lua/5.1/kong/cmd/utils/inject_confs.lua:85: in function
[C]: in function 'xpcall'
/usr/local/bin/kong:99: in function 'file_gen'
init_worker_by_lua(nginx.conf:117):48: in function
[C]: in function 'xpcall'
init_worker_by_lua(nginx.conf:117):55: in function
```

kong version 3.6.1
postgres 13.0

This is me running the docker command:
docker run --rm --network=kong-net -e "KONG_DATABASE=postgres" -e "KONG_PG_HOST=8.0.88.64" -e "KONG_PG_USER=kong" -e "KONG_PG_PASSWORD=kong12345" -e "KONG_CASSANDRA_CONTACT_POINTS=kong" kong:latest kong migrations bootstrap

Any helps, please!
Thank you.
Fran

Start by reproducing the reported `kong migrations bootstrap` Docker command with the listed PostgreSQL settings and inspect the failure at `kong/cmd/utils/inject_confs.lua:28`. Compare the resulting error with the Kong 3.6.1 and PostgreSQL 13.0 versions reported. Done means the migration bootstrap completes successfully against the external PostgreSQL database, with the cause documented.

Written by the indexing model from the issue text.