Add support for custom backup provider patterns

[mrwogu]

Overview

This PR adds the ability to extend backup providers with custom patterns using environment variables, making the tool more flexible for users who have specific container naming conventions.

Features

• Added support for custom backup provider patterns via environment variables
• Extended the documentation with details on how to use this feature
• Added tests to ensure functionality works correctly

Implementation

Users can now add custom container patterns to the existing backup providers by setting environment variables in the format CUSTOM_BACKUP_PROVIDER_<provider_name>_PATTERNS=pattern1,pattern2,.... This allows for extending the patterns list without modifying the code.

For example:

CUSTOM_BACKUP_PROVIDER_POSTGRES_PATTERNS=immich-app/postgres,custom-postgres

Documentation

Updated the README.md with:

• A new "Custom Backup Providers" section explaining the feature
• Example usage with environment variables
• Updated docker-compose.yml example

Testing

• Added a comprehensive test for the custom backup provider patterns feature
• All existing tests continue to pass

Use Case

This addresses the use case where users need to back up databases with container patterns not included by default, such as "immich-app/postgres".