What is docker?
Docker is a way to enforce the correct dependencies are installed, every single time you setup a server. It saves a lot of time, however needs some setup.
The most important thing however, if you setup a docker file properly, you will be able to perform as much setup as you want upon a server automatically. It can save a lot of time, however requires you to learn a fair bit. Luckily, most of that learning is optional, only if you want to make your own. Instead you can use docker containers other people have made.