[BUG] Bug #46: Environment variables REDIS_* are not applied to site_config.json

[noseshimself]

Describe the bug
See #46

Due to the lack of a mechanism equivalent to the erpnet-configure container of frappe/frappe_docker the contents of the environment variables

REDIS_CACHE_HOST=redis:6379/3
REDIS_QUEUE_HOST=redis:6379/4
REDIS_SOCKETIO_HOST=redis:6379/5

I don't understand why there have to be multiple containers with identical redis servers instead of using the same server for all three containers this is not working as expected. It's also required for a redundant setup where the redis server might not be running on the same host.

To Reproduce

Steps to reproduce the behavior:

1. Use a modified docker-compose.yml that is making use of the environment variables

REDIS_CACHE_HOST=redis:6379/3
REDIS_QUEUE_HOST=redis:6379/4
REDIS_SOCKETIO_HOST=redis:6379/5

2. Launch the containers accessing redis servers
3. Find the error described in [BUG] Redis connection to localhost:12000 failed and redis.exceptions.ConnectionError: Error 99 connecting to localhost:11000. Cannot assign requested address #46

Expected behavior
Being able to specify a different redis server

Additional context
I know that "works with frappe/frappe_docker" is a weak argument but somehow fits it.
It is if course possible to edit