Many at Once (compose & networking)

The last board of the Docker arc: one real app is never one container, and this is the file that wires several of them together, plus the exact place that file stops being enough.

Before you start. An image is the frozen box the last lesson built, docker run starts one, and its flags are the extra words on that command that punch holes in the box, like -p to open a port so the outside can reach it. This lesson is what happens when an app needs four of those boxes, talking to each other, instead of one.