Enable deletion of hidden files (#1061)

If this is not done, then once we exit the docker container, the hidden dirs will be owned by root and will not be deletable by the Jenkins process

Deletion process from: https://unix.stackexchange.com/questions/77127/rm-rf-all-files-and-all-hidden-files-without-error/77196

vars/simpleDockerNode.groovy

@@ -115,7 +115,7 @@ def call(String dockerImageName, String dockerRegistryUrl, String registryCreden
                             // but this is unlikely.  This will probably eventually be fixed on the workspace cleanup
                             // side of things, so don't worry about it too much
                             echo "Cleaning workspace ${WORKSPACE}"
-                            sh 'rm -rf *'
+                            sh 'rm -rf ..?* .[!.]* *'
                             // Use the workspace cleaner now.
                             cleanWs deleteDirs: true, patterns: [[pattern: '**/*', type: 'INCLUDE']]
                         }