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Containers

Learning Objectives
  • List the benefits of containerisation
  • Run Docker containers locally
  • Manage multiple containers with Docker compose.
  • Use Docker compose to communicate between containers.

Pre work​

Set up​

If nobody has done so already, copy and paste this command into Slack.

/prework Containers
Task

Install Docker Desktop on Mac Silicon using Homebrew:

brew install --cask docker

After installation, open Docker Desktop and verify it is running. Then confirm the installation by running:

docker run hello-world

You should see a "Hello from Docker!" message.

Research - Containers​

Research - containers

Find answers to the following questions:

  • What is Docker?
  • What is "containerisation"?
  • What are the benefits of containerisation?
  • At a high level, how does Docker work?

Experiment​

Task

Run the following commands in your terminal and observe the output carefully. What can you work out about what is happening? Which commands make persistent changes?

  • docker ps -a
  • docker image ls
  • docker run hello-world

Workshop​

Warm-up​

Put the trainees into pairs

Tell the trainees that they should teach each other their understanding of what a Container is, what it's benefits are and roughly how they work

Containers​

Introduce the concept of a Kernel: the core part of an operating system that manages communication between software and hardware (CPU, memory, devices). Every operating system has one.

Explain what a Virtual Machine (VM) is: a complete, isolated computer running inside your physical computer. A VM includes its own operating system (kernel and all), and uses a program called a hypervisor to share the underlying hardware. VMs are isolated and portable, but they are large and slow to start because each one carries a full OS.

Explain what a Container is: similar to a VM in that it packages up an application with everything it needs to run, but instead of including a full operating system, containers share the host machine's kernel. This makes containers much lighter, faster to start, and more efficient than VMs.

Summarise the progression: hardware → OS kernel → VM (full OS on top) → container (shared kernel, isolated process).

Ask trainees to explain what 'containerisation' is, and why it is useful.

Introduce Docker as one example of a tool used for containerisation.

Check that all trainees have Docker installed and can run Docker Desktop.

Introduce key terminology:

  • Image (a blueprint for an application and all its dependencies)
  • Container (an application built from a container)

Command-line Docker​

Demonstrate how to run a simple Ubuntu container in the terminal:

docker run -it ubuntu

Talk through the process of pulling the image and spinning up the container.

Show that the container can be interacted with just like a normal computer.

Switch to another terminal and show the running container with docker ps.

Highlight key details:

  • The randomly-assigned name
  • The container status

Show trainees Docker hub, and all the different images they can build containers from.

Stop the Ubuntu container and run a Python one instead.

docker run -it python

Explore, with trainees, the options for viewing stopped containers (docker ps -a) and starting them again (docker run -i [name]).

Mirror the same options in Docker Desktop.

Docker compose​

Point out that some Docker commands can get quite lengthy:

docker run --name example-container -p 3000:3000 ubuntu

Explain the above command - an Ubuntu container with a specific name that has its port 3000 mapped to 3000 on the local machine, allowing communication with the container from outside.

Wonder, aloud, if there is some easier-to-type/-organise solution.

Introduce the idea of a compose file.

services:
  comp:
    image: ubuntu
    container_name: example-container
    ports:
      - 3000:3000

Explain that using compose files has two benefits:

  • Complex shell commands can be written in a more maintainable way
  • Multiple different containers can be started with the same command

Demonstrate starting (docker compose up) and stopping (docker compose down) composed containers.

Demonstrate that you can add postgres to your compose.yaml like so

services:
  comp:
    image: ubuntu
    container_name: chris_ubuntu_compose
    ports:
      - 3000:3000
  database:
    image: postgres
    container_name: chris_postgres
    environment:
      - POSTGRES_PASSWORD=verysafepassword
    ports:
      - 4768:5432

Demonstrate that you can then connect to the database

psql -d postgres -h localhost -p 4768 --u postgres

Conclusion​

Mention that, in future weeks, trainees will learn to create their own Docker images.

Stress that, for now, they need to be comfortable with the basic idea and able to start containers from provided scripts.

Allow sufficient time for 'how do I...' questions.