Docker Basics in 30 seconds

• Basic Terminology
• Docker Commands
  • Working with images
  • Working with containers

When I first started learning Docker, it was really baffling for me. I thought I understood Linux and it’s terminology pretty well, but Docker really threw me for a loop. In hindsight it feels pretty silly, but at the time it was serious and I was lost. So this is a quick primer for anyone else who is learning Docker and confused by all the new words.

Basic Terminology

So it’s best to think of the basic parts of Docker as if they were real physical things, like a picture or a sketch that you drew.

• Image - Using the sketch analogy, this is the sketch you drew. You can photocopy this sketch as many times as you need. More technically, an image is just a pre-built system. Nine times out of ten, it’s a complete OS, generally with something special installed. For example, the MariaDB images are just Ubuntu with MariaDB already installed.
• Container - A container is a copy of your image that you can actually play with. You take your sketch and you photocopy it over and over. Every copy you make you modify somehow. Maybe you color it with pens. Cut pieces off of it. Maybe on one you use acrylics, and on another you use watercolors. Regardless, you can make as many copies as you want, and every copy starts off exactly the same. So in Docker, containers work exactly like this. You download the image from DockerHub, you install some packages, add some users, maybe modify some config files, and boom you now have a working virtual machine to use.
• Dockerfile - This is the “code” you write to build an image which eventually becomes a container. A Dockerfile is where you tell Docker what OS you want to start with (generally, you will start with a pre-existing OS container you pull from Dockerhub). You can also add files to it, run scripts, modify configuration, install packages, expose ports, etc. When someone asks if you have experience with Docker, the ins and outs of Dockerfiles is most likely what they’re going to quiz you on.

Docker Commands

Working with images

• Pull - This pulls down and an existing image from DockerHub or from another repository of your choice.
• Build - This builds the image, using a Dockerfile, that you can use to create containers from.

Working with containers

• Create - Creates a new container from an existing image. It does not start it though. If you check the status of your containers, your newly created container will show the status of “created.”
• Start - This starts your container. With physical computers you would call this boot or something similar, but start actually makes more sense in this context (in my opinion).
• Run - This one really confused me, because all the tutorials for Docker use this command and leave out the previous two. This both creates your container and starts it in one command.