Docker Basics Part 1

Images & Containers

Images

• Images are one of the two core building blocks Docker is all about (the other one is "Containers").
• Images are blueprints / templates for containers. They are read-only and contain the application as well as the necessary application environment (operating system, runtimes, tools, ...).
• Images do not run themselves, instead, they can be executed as containers.
• Images are either pre-built (e.g. official Images you find on DockerHub) or you build your own Images by defining a Dockerfile.
• Dockerfiles contain instructions which are executed when an image is built ( docker build . ), every instruction then creates a layer in the image. Layers are used to efficiently rebuild and share images.
• The CMD instruction is special: It's not executed when the image is built but when a container is created and started based on that image.

Containers

• Containers are the other key building block Docker is all about.
• Containers are running instances of Images. When you create a container (via docker run ), a thin read-write layer is added on top of the Image.
• Multiple Containers can therefore be started based on one and the same Image. All
• Containers run in isolation, i.e. they don't share any application state or written data.
• You need to create and start a Container to start the application which is inside of a Container. So it's Containers which are in the end executed - both in development and production.

Key Docker Commands

For a full list of all commands, add --help after a command - e.g. docker --help , docker run --help etc. Also view the official docs for a full, detailed documentation of ALL commands and features: https: //docs.docker.com/engine/reference/run/

Important: This can be overwhelming! You'll only need a fraction of those features and commands in reality!

• docker build .: Build a Dockerfile and create your own Image based on the file
  • -t NAME:TAG: Assign a NAME and a TAG to an image
• docker run IMAGE_NAME: Create and start a new container based on image IMAGENAME (or use the image id)
  • --name NAME: Assign a NAME to the container. The name can be used for stopping and removing etc.
  • -d: Run the container in detached mode - i.e. output printed by the container is not visible, the command prompt / terminal does NOT wait for the container to stop
  • -it: Run the container in "interactive" mode - the container / application is then prepared to receive input via the command prompt / terminal. You can stop the container with CTRL + C when using the -it flag
  • --rm: Automatically remove the container when it's stopped
• docker ps: List all running containers
  • -a: List all containers - including stopped ones
• docker images: List all locally stored images
• docker rm CONTAINER: Remove a container with name CONTAINER (you can also use the container id)
• docker rmi IMAGE: Remove an image by name / id
• docker container prune: Remove all stopped containers
• docker image prune: Remove all dangling images (untagged images)
  • -a: Remove all locally stored images
• docker push IMAGE: Push an image to DockerHub (or another registry) - the image name/ tag must include the repository name/ url
• docker pull IMAGE: Pull (download) an image from DockerHub (or another registry) - this is done automatically if you just docker run IMAGE and the image wasn't pulled before