Collect static files during deployment

[agoncharov-reef]

During standard updating of frontend files and redeploying, I noticed that app was using old static files instead of new ones - until I manually ran collectstatic and restarted the app.

This happens because:

• we collect static files during image build; these files are "baked" into resulting docker image
• we have backend_static volume in docker-compose which is shared between app and nginx containers
• that backend_static volume is not available during image build, so static files inside it are not updated

Proposal:

• don't collect static files during image build
• collect static files during deployment - by running temporary instance of app and collecting files into backend_static volume