title: ignore all .git folders in .dockerignore
body: <p>I have a project that have a few git clones, every folder will have his .git and some folder has clones inside folders</p>
  
  <p>So it's possible to ignore all .git folders?</p>
  
  <p>I tested many rules as described in the docs, but I only managed to ignore on the top folder.</p>