[FR] Common vars

[hedhyw]

Please describe the problem / need you are trying to solve.
Now, If I need to change the same values in many places.
Example: "POSTGRES_PASSWORD".
Because of this, an incorrect configuration is possible.

Describe the feature or the improvement you'd like and what are you trying to achieve.
Have common vars in .env file: https://docs.docker.com/compose/environment-variables/
Also, it will be good to create a script, that will generate .env file with good secrets for a given domain.

Describe alternatives you've considered
Common vars at the top of the docker-compose file, which will be used not only in the back but also in other services.

Additional context
I can create a PR for this after consideration.