Docker: Implement persistent storage for PSQL

[kyleecodes]

Resolves #588 & implements persistent storage.

N/A

What changes did you make and why did you make them?

• In a previous update, the backend's dependency overhead was reduced by moving the PostgreSQL database into Docker. This pull request introduces persistent storage by configuring Docker volumes / bind mounts for the PostgreSQL container. With this change, database data will persist even if containers are deleted, ensuring that data remains intact across container restarts. This also eliminates the need for maintainers to manually populate databases in cloned forks when reviewing forked pull requests. The named volume postgres-data ensures cross-compatibility across all operating systems.
• .env now loaded in docker-compose
• Respective docs are updated to reflect the change.

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