fix: Add a check for existing docker db volume

[mr-karan]

Fixes #517

PostgreSQL DB container uses initdb to initialize the DB data directory. The password/username/DB name is set during this time. If any of these values change, these don't get updated automagically since initdb runs only once on an absence of data directory.

So, if the install-prod.sh script is cancelled midway, the listmonk-data docker DB volume is already created. If the script is run again, it will generate a new password and set this in docker-compose and config.toml. However, this password is wrong, since the listmonk-data DB directory was initialized with the script's previous run's password.

This script simply checks for an existence of listmonk_listmonk-data volume and if it exists, errors out immediately. It is upto the user to continue (by manually changing password in config.toml/docker-compose.yml) or remove the volume (by docker-compose down -v

➜ ./install-prod.sh

>  checking for an existing docker db volume
x listmonk-data volume already exists. Please use docker-compose down -v to remove old volumes for a fresh setup of PostgreSQL.