use single quotes for string without interpolation

Jenkinsfile

@@ -7,14 +7,14 @@ import com.cloudogu.ces.cesbuildlib.*
 node('docker') {
 
   // Change this as when we go back to default - necessary for proper SonarQube analysis
-  mainBranch = "2.0.0-m3"
+  mainBranch = '2.0.0-m3'
 
   properties([
     // Keep only the last 10 build to preserve space
     buildDiscarder(logRotator(numToKeepStr: '10')),
     disableConcurrentBuilds(),
     parameters([
-        string(name: 'dockerTag', trim: true, defaultValue: "latest", description: 'Extra Docker Tag for cloudogu/scm-manager image')
+      string(name: 'dockerTag', trim: true, defaultValue: 'latest', description: 'Extra Docker Tag for cloudogu/scm-manager image')
     ])
   ])
 
@@ -69,7 +69,7 @@ node('docker') {
           docker.withRegistry('', 'hub.docker.com-cesmarvin') {
             image.push(dockerImageTag)
             image.push('latest')
-            if (!"latest".equals(params.dockerTag)) {
+            if (!'latest'.equals(params.dockerTag)) {
               image.push(params.dockerTag)
 
               def newDockerTag = "2.0.0-${commitHash.substring(0,7)}-dev-${params.dockerTag}"
@@ -102,7 +102,7 @@ String mainBranch
 
 Maven setupMavenBuild() {
   // Keep this version number in sync with .mvn/maven-wrapper.properties
-  Maven mvn = new MavenInDocker(this, "3.5.2-jdk-8")
+  Maven mvn = new MavenInDocker(this, '3.5.2-jdk-8')
 
   if (isMainBranch()) {
     // Release starts javadoc, which takes very long, so do only for certain branches