Disable concurrent builds to reduce memory usage

As described in the issue #1343 builds get killed by the OOM killer
sometimes.

Add lock('iis-build') to the build options to reduce
memory usage whem multiple builds are triggered.
As described eg. in https://issues.jenkins.io/browse/JENKINS-35359
the disableConcurrentBuilds option can only prevent concurrent builds
of a single branch and using locks is the recommended workaround
for serializing all the builds in a multibranch pipeline.

Jenkinsfile.dockerized

@@ -12,6 +12,7 @@ pipeline {
     options {
         buildDiscarder(logRotator(numToKeepStr: '5'))
         timeout time: 60, unit: 'MINUTES'
+	lock('iis-build')
     }
 
     environment {