Get session secret from env instead of hardcoding

docker-compose.dev.yml

@@ -17,6 +17,7 @@ services:
                         - ~/.cache:/.cache
                 environment:
                         BUDGETEER_DB: postgres://budgeteer:budgeteer@db:5432/budgeteer
+                        BUDGETEER_SESSION_SECRET: random string for JWT authorization
                 depends_on:
                         - db