What happens when you deploy your app, and then you want to fix a bug, or simply release an update?
Once you have a container running, you can get its ID using
docker ps, then you use
docker commit to create a new tag for the image from it:
docker commit <id> <username>/<imagename>:<tagname>
You can also add a changelog message that lists what changed in the new version:
docker commit --change "description" <id> <username>/<imagename>:<tagname>