Introduce "dev environment" and dotenv

There are now two dockerfiles. One for production, one for development.
All configuration is now also handled through dotenv files,
which these dotenv files, as well as the included VSCode launch tasks
use.

.vscode/tasks.json

@@ -12,7 +12,7 @@
         {
             "label": "mariadb",
             "type": "shell",
-            "command": "docker run --rm --name some-mariadb --env MARIADB_USER=devuser --env MARIADB_PASSWORD=devpw --env MARIADB_ROOT_PASSWORD=devrootpw --env MARIADB_DATABASE=karaoqueue -p 3306:3306 mariadb:latest",
+            "command": "docker-compose -f docker-compose.yml up --remove-orphans",
             "isBackground": true,
             "activeOnStart": false
         }