Outline

• In this lecture, you will learn:
  • The basics of Docker containers
  • The practical usage of important Docker commands

Why Learn Containers?

A Solution with Containerisation

Reminder: What is Containerisation?

• A container is a standard unit of software that packages up code and all its dependencies so that the application runs quickly and reliably from one computing environment to another.
• You probably know:
  • Container ≠ VM
  • Share Host OS
  • An isolated process
  • Motto: Build, ship and then run it anywhere

Key Benefits of Using Docker Containers

Key Docker Terminology

• Docker Image: The basis of a Docker container. Represents a full application
• Docker Container: The standard unit in which the application service resides and executes
• Docker Engine: Creates, ships and runs Docker containers deployable on a physical or virtual, host locally, in a datacenter or cloud service provider
• Registry Service: Cloud or server based storage and distribution service for your images

Central Dogma of Containerisation

Docker Architecture

Important Docker Commands

Referencing an Image from a Registry

• library: Pull an image from Sylabs cloud library
  • library://user/collection/image:tag
• docker: Pull an image from Docker Hub
  • docker://repository/image:tag
• Quay.io : Pull an image form RedHat Quay.io registry
  • docker: //quay.io/repository/image:tag
• shub: Pull an image from Singularity Hub
  • shub://repository/image:tag

Pulling an Image from a Registry

• Usage: docker pull [OPTIONS] NAME[:TAG|@DIGEST]
• Example:

• Example with a specific tag: docker pull ubuntu:23.10

Running Docker Containers: A Toy Example

More on Running Docker Containers

• Usage: docker run [OPTIONS] IMAGE [COMMAND] [ARG…]
  • docker run … # simple run
  • docker run -d … # run in detached mode (= in the background)
• Example:
  

Running Docker Containers in an Interactive Mode

• Usage: docker run -it IMAGE[:TAG|@DIGEST] [COMMAND] [ARG…]
• Containers are non-interactive by default
• Example:
  

Running Status of Containers

• Usage: docker ps # list all running containers
• Example:
  
• Usage: docker ps -a # list all running and stopped containers

Entering and Exiting Out of Containers

• Usage: docker exec [OPTIONS] CONTAINER COMMAND [ARG…]

• Example:
  

• Just to come out of container, use exit or control+d

• Save and exit out of container, use control + p and then control+q

Running a Command in an Existing Container

• Usage: docker exec < options > < container ID> < command >
• Container ID/name should be from a running container
• Example:
  

Create a New Image From a Modified Container

• Usage: docker commit [OPTIONS] CONTAINER [REPOSITORY[:TAG]]
  • Create an image of an edited container on local system
• Example:
  

Containers in Docker Archive Files

• Docker allows saving images into single file as tar archives
• Tar archives contains layers and metadata that make up a Docker container
• One can convert the tar file into an Apptainer container using the docker-archive bootstrap agent
• Example:
  apptainer build local_tar.sif docker-archive:local.tar