Enable caching

[xenophonf]

See also

• https://flask-caching.readthedocs.io/
• https://github.com/SvenMeiLing/myflasky/blob/master/app/create_app.py (particularly the with @app.app_context(): code block)
• https://github.com/opf/openproject-deploy/blob/stable/13/compose/docker-compose.yml