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.