fix: add DATABASE_URL to .postgres env template to ensure it's available in all container contexts fix #6273

[shriramThakare3]

Description

I have added an explicit DATABASE_URL environment variable to the .env template. This ensures the connection URI is available in all container contexts (including docker exec sessions) rather than being dynamically exported only during the entrypoint execution.

I also added a single-line comment to the template to ensure developers keep the URI in sync with the individual POSTGRES_USER and POSTGRES_PASSWORD variables.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

The current implementation constructs DATABASE_URL within entrypoint.sh. While this works for the primary container process, it does not persist for subsequent exec sessions or external debugging tools.

By moving this into the environment template:

• Persistence: The variable remains available throughout the entire container lifecycle.
• Clarity: Other developers can see exactly how the URI is constructed without digging into shell scripts.
• Consistency: Using the same placeholders ensures that the user fills in credentials consistently across all fields.