update to version 18

parameterized nextcloud version in build files

dockerfiles/services/nextcloud/Jenkinsfile

@@ -2,6 +2,7 @@ pipeline {
     environment {
         APP_NAME = 'xaibox'
         WORKDIR = 'dockerfiles/services/nextcloud'
+        NC_VERSION = 18
     }
 
     agent { label 'docker' }