OSB-45: missing protocol

LCU/Maintenance/Docker/Jenkinsfile

@@ -15,7 +15,7 @@ pipeline {
                 echo 'building tag ${env.DOCKER_TAG}'
                 
                 script {
-                   withDockerRegistry(credentialsId: 'nexus', url: 'nexus.cep4.control.lofar:18081') {
+                   withDockerRegistry(credentialsId: 'nexus', url: 'http://nexus.cep4.control.lofar:18081') {
                         dir ('LCU/Maintenance/Docker/BaseMaintenance/'){
                             docker.build "lofar-maintenance-base:${DOCKER_TAG}"
                         }