Jenkins: Make sure Docker images are fresh

Jenkinsfile

@@ -20,6 +20,7 @@ pys.each { py ->
             stage("Prepare docker $py.name") {
                 dir('dockerbuild') {
                     deleteDir()
+                    docker.image(py.docker).pull()
                     buildDockerfile(py.docker)
                     image = docker.build("dosage-$py.docker")
                 }
@@ -98,7 +99,9 @@ def windowsBuild() {
 def windowsBuildCommands() {
     deleteDir()
     unstash 'bin'
-    docker.image('tobix/pywine').inside {
+    def img = docker.image('tobix/pywine')
+    img.pull()
+    img.inside {
         sh '''
             . /opt/mkuserwineprefix
             tar xvf dist/dosage-*.tar.gz