Add & update stuff supporting debugging

[trkks]

• Remove old environment variable -check left in Dockerfile + switch it to a new one
• Add config for starting debugging of the main application in VSCode
• Python IDE features should no longer complain about import paths
• NOTE: Remove unnecessary-seeming line in host_app/__init__.py that prevented running the app in some situation

Especially the last bullet should probably be checked to not break anything on anyone elses setup!