Lets not download the gravatar and then download a different one after the js is triggered

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)

docs/theme/mkdocs/header.html

@@ -32,7 +32,7 @@
       <ul class="nav user">
         <li class="dropdown">
           <a id="logged-in-header-username" class="dropdown-toggle" data-toggle="dropdown" href="#">
-            <img class="profile" src="https://secure.gravatar.com/avatar/26dc2b32b9e753823aef55e89687a9fc.jpg?s=30&amp;r=g&amp;d=mm" alt="profile picture">ostezer
+            <img class="profile" src="" alt="profile picture">
           </a>
           <ul class="dropdown-menu pull-right">
             <li><a href="https://hub.docker.com/">View Profile</a></li>