Update docker-compose.yml & documentation

• docker-compose.yml: Reorder environment variables lexicographically & add a new one for the http path
• Documentation update to reflect keycloak v17+
• Additionally, this MR aims to deploy keycloak to v18 (set through the CI variables)

.example.env

@@ -9,12 +9,13 @@ KC_TAG=20.0
 KC_DEV_MODE=1
 KC_ADMIN_USER=admin
 KC_ADMIN_PASSWORD=u_will_never_guess
+KC_HTTP_RELATIVE_PATH=/auth
 
 # db backups
-BACKUP_SCHEDULE="@every 6h"
 BACKUP_DIR=/db-backups
 BACKUP_KEEP_DAYS=7
 BACKUP_KEEP_WEEKS=4
 BACKUP_KEEP_MONTHS=6
+BACKUP_SCHEDULE="@every 6h"
 BACKUP_SUFFIX=.sql.gz
 BACKUP_TZ=Europe/Vienna