title: updating docker image given changes to local filesystem
body: <p>I am trying to work out how I can update an existing image when I make changes to the local filesystem that was used to create the docker image. I thought that I could use docker commits to do that, but it seems that that allows you to change the image when there are changes to the filesystem on a running image? </p>
  
  <pre><code> /app.py 
  </code></pre>
  
  <p>build from file system </p>
  
  <pre><code> sudo docker build -t app 
  </code></pre>
  
  <p>now there are local changes to /app.py. How do I change the image app to reflect the changes to /app.py? right now I'm having to delete the old image and then create a new one. </p>
  
  <pre><code>sudo docker rmi app 
  sudo docker build -t app
  </code></pre>
  
  <p>any help is appreciated!</p>