docs: add precise steps for updating pure Docker instances

Created by: slimsag

@sourcegraph/distribution please review ASAP. This document will be used from now on to communicate all changes in pure-Docker instances that admins must perform.

In the future, I intend to create a similar file in this directory for Kubernetes, Server, and Docker-Compose deployments.

Effectively, I want all site admins to be confident in their upgrades by knowing they are following an exact set of instructions for upgrading their instance -- including any manual migrations or steps that are needed.