It was mostly how we wanted to use it at my company. It didn't seem clear whether docker should just set up the system (apt-get, pip install) or also checkout the code. To add to the complexity, we have a puppet script we run on the machine to install dependencies. I had no idea how Docker who supposed to fit into this pipeline. So basically, I didn't know what docker was supposed to be good at, i.e. do better than other tools.