What is Docker?
Docker is containerizing technology that allows easy packaging of apps into containers. This solves the problem of deployment. With Docker you can type a single command into your console and your application can be downloaded and installed. What's more, your application be downloaded and installed in an independent copy of the operating system with an independent file system. This environment will still run off of the main Linux kernel which is more efficient than running a virtual private server, a VPS, for each site.
Docker prefers that you deploy one service per container, however this is not a hard requirement. You may deploy more than one service per container if it makes sense for the specific use case. In this sense it can be used similarly to a VPS. It allows you to isolate one or more processes in a self contained environment. The Docker way of doing things is different from the way that we did things and it is in many ways better.