Enable image diffs for Eclipse ESCET GitLab

GitLab supports image diffs. See e.g. https://gitlab.eclipse.org/eclipse/dash/org.eclipse.dash.handbook/-/commit/2abbe6e5fe35ced19b15823889aa5496fe2fb896. However, they don't work for the Eclipse ESCET GitLab. We should investigate why that is. My first guess would be that it has something to do with .gitattributes configuration.