I found that docker insert will replace directories in the image. For example, running

% docker insert b6be9bf172a2 http://localhost:8000/myfile.txt /home

will replace /home in the image with myfile.txt. I naively assumed cp behavior, where the file would end up inside /home, not replace it. Now I know. But it might be better to prevent this kind of destruction. (I can imagine what would have happened if I had typed / instead of /home.)