Send X-Docker-Endpoints at the end of a push

    <kencochrane> It looks like somewhere recently we stopped sending the registry server 
    that the images were uploaded to back to the index on the last step of the push
    <kencochrane> so basically in the index we don't know which registry the images were 
    pushed too.. not a big deal now since we only have one registry, but as we add multiple it 
    will cause problems. also makes it harder for me when I need to delete repos since I don't 
    really know which registry has the repositories.

cc @kencochrane