title: How to commit a docker container with null entrypoint?
body: <p>I have an origin docker image, I want to ran a container and modify it, then commit a new image.
  I inspect the original docker image, and it has a conf</p>
  
  <pre><code>"Config": {
          ...
          "Cmd": [
              "/bin/bash",
              "-ic",
              "./run.sh"
          ],
  
           ...,
          "Entrypoint": null,
          ...
      },
  </code></pre>
  
  <p>I want to keep the same Cmd and Entrypoint for the new committed image.
  But no matter I use --change or not, I cannot keep the "Entrypoint": null for the new image. Is there any way to do so?
  Moreover, I also use docker-py, it has the same problem.</p>