docs: include Postgres container + network in docker run guide

[LarytheLord]

Summary

• document that plain docker run installs require a separate PostgreSQL container
• add a minimal Postgres docker run example (db_recipes)
• update the app docker run example to join the same Docker network

Why

The current docker run section sets POSTGRES_HOST=db_recipes but does not show creating a DB container or shared network. This can lead to startup failures where the app cannot reach the database.

Closes #4283

[CLAassistant]

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