[Misc] Revert of faulty code improvement and introduction the image plugin docker tests

...ckeditor-plugins/src/main/resources/xwiki-image/plugin.js

...docker/src/test/it/org/xwiki/ckeditor/test/ui/AllITs.java

...src/test/it/org/xwiki/ckeditor/test/ui/ImagePluginIT.java

...itor-test-docker/src/test/resources/ImagePlugin/image.gif

...tor-test/xwiki-platform-ckeditor-test-pageobjects/pom.xml

...ts/src/main/java/org/xwiki/ckeditor/test/po/CKEditor.java

...java/org/xwiki/ckeditor/test/po/ImageDialogEditModal.java

...va/org/xwiki/ckeditor/test/po/ImageDialogSelectModal.java

...ava/org/xwiki/index/tree/test/po/DocumentTreeElement.java