title: Remove existing image when using `docker commit`?
body: <p>I have a script which I use to build docker images. The script starts up a docker container, executes some commands on it, and then does <code>docker commit</code> to fixturize the image. When I commit with an image name/tag, and then later commit with the <em>same</em> image name/tag, I would like the previous image to be removed, since at that point it's just taking up disk space. Instead it sticks around (I can see it in <code>docker images</code>, with its repository and tag both listed as <code>&lt;none&gt;</code>). Is there a way to have docker automatically remove these replaced images?</p>