The payload is anything which can be recorded on the filesystem by running a unix command. For example:

     # Run command which adds your payload
     $ docker run base apt-get install curl
     5b4a1ee8

     # Commit the result to a new image
     $ docker commit 5b4a1ee8 nwg/base-with-curl

     # Run a command from the new image. Your payload is available!
     $ docker run nwg/base-with-curl curl http://www.google.com

Docker doesn't care how the payload was added. apt-get, shell script, pip install, gem bundle... All the same. In the end it's just a change to the filesystem.