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Booker Ochieng
Booker Ochieng

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Getting started with docker

#devops #docker #containers

What are containers?

  • Containers are a way of packaging applications together with all required dependencies and configuration, making the application easy to move, share, and deploy.
  • Before containers, developers working on different operating systems had to manually install all required dependencies to run a project. This setup process could sometimes fail or cause inconsistencies between environments. For example:
    • "A React project requires Node.js and project-specific dependencies."
  • With containers, once the project is packaged, only one tool is required — a container runtime such as Docker — which includes all necessary configurations and dependencies.

Container vs image

  • Image – A packaged, read-only snapshot of an application with its dependencies and configuration.
  • Container - Is the outcome of starting/running an image.

Docker vs Virtual Machine

  • A OS is made up of the following layers :
    1. Application layer
    2. Kernel
    3. Hardware

Docker

  • It virtualizes the OS Application layer resulting in small bundle size
    • Services and apps bundled into a docker container run on top of the application layer
    • As a result, it uses the kernel of the host machine as it lacks its own
  • Docker containers use the Linux kernel. On macOS and Windows, Docker runs inside a lightweight Linux virtual machine.

Virtual Machine

  • Virtual machines can run any operating system, regardless of the host OS, because they virtualize hardware.

Docker Installation

  • Installation details for each os can be found from the docker website

Linux Installation Tip

  • Once you've installed docker, to avoid having to use sudo each time you run a docker command, add your user to the docker group : 
sudo usermod -aG docker ${USER}
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Once done log out and log in again.

  • Alternatively you could change the owner of the /var/run/docker.sock file to your user ie 
sudo chown $USER /var/run/docker.sock
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This approach however doesn't persist across restarts since when docker starts this file is recreated and ownership resets.

Common docker commands

1 . List out all images

docker images
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2 . Start a container from an image

# Run in attached mode ie Ctrl+C stops the proces
docker run redis:latest
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# Run in detached mode ie Ctrl+C does not stop the proces as it runs in the background
docker run -d redis:latest
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# --name <someName> -> assigns a name to your container
# -p <exposedPort>:<internalPort> -> Used to specify that the app 
#  running in container at the port "<internalPort>" should be 
#  exposed to the host at this port "<exposedPort>"
docker run --name mongodb-4.4 -p 27017:27017 mongo:4.4
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3 . List out containers and their metadata(container id, names etc)

docker ps # list only running containers
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docker ps -a # list all container even ones that got stopped
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4 . Stop container

docker stop <container-id>
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5 . Start a stopped container

docker start <container-id>
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