Add Gunicorn-friendly app factory and document Compose deployment

[rogerlew]

• introduce rq_dashboard/app.py providing a Flask application factory that wraps the existing blueprint, normalises the /rq-dashboard prefix, pulls env configuration (Redis URL, serializer, etc.), and exposes a callable for Gunicorn and other WSGI servers
• update the README with a “Running as Container in Docker Compose” guide covering pros/cons versus embedding the blueprint, the custom app.py rationale, an inline Dockerfile and compose service example, and Caddy reverse-proxy headers needed to serve the UI under /rq-dashboard/

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes # (issue)

Type of change

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• Documentation Change
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have run tests (pytest) that prove my fix is effective or that my feature works
• I have updated the CHANGELOG.md file accordingly
• I have added tests that prove my fix is effective or that my feature works

[cjlapao]

@rogerlew Thanks for the work, this has now been released