New pwpush-worker Docker container

[pglombardo]

Description

This PR adds a new docker container pwpush-worker that can be run to execute periodic background tasks.

It should be run with an identical configuration to the main pwpush container. This includes any mounted filesystems.

Related Issue

#468

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created. (rake test)
• I've added documentation as necessary so users can easily use and understand this feature/fix.