Docker Primer

When we think of virtualization today, we may think of Virtual Box, which abstracts away the system processes, and lets you run a completely system from another. Think of Docker as Virtual Box, but extremely lightweight (in terms of resource consumption). Obviously I’m over simplifying the explanation a little, and a whole of things are getting lost in simplification. But for now, this will do.

Installation

  • Install docker
apt remove docker docker-engine docker.io
apt update && apt -y upgrade
apt install -y linux-image-extra-$(uname -r) linux-image-extra-virtual #FOR UBUNTU 18: apt install -y linux-image-$(uname -r) linux-image-extra-virtual
apt update && apt -y upgrade
apt install -y apt-transport-https ca-certificates curl software-properties-common
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add -
add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" #FOR UBUNTU 18: instead of stable use edge (just for now, since they haven't released the stable version on 18.04 yet)
apt update && apt -y upgrade
apt install -y docker-ce
  • Install docker-compose
curl -L https://github.com/docker/compose/releases/download/1.21.2/docker-compose-$(uname -s)-$(uname -m) -o /usr/local/bin/docker-compose #use the latest docker-compose version, by looking at releases on github pages [https://github.com/docker/compose/releases]https://gist.githubusercontent.com/mamigot/1abdefc684f97d3476f177d85248ff36/raw/5f57b4918b237f8e0127b45984fa51a483218349/docker-compose.yml
chmod +x /usr/local/bin/docker-compose

Post Installation stuff

Allow docker usage without sudo access

The docker daemon binds to a Unix socket instead of a TCP port. By default that Unix socket is owned by the user root and other users can only access it using sudo. The docker daemon always runs as the root user. To prevent using sudo when you use the docker command, create a Unix group called docker and add users to it. When the docker daemon starts, it makes the ownership of the Unix socket read/writable by the docker group

sudo groupadd docker
sudo usermod -aG docker $USER
newgrp docker

Terminology

  • Images

Images are read only We upload images to hub. Think of them as classes in an object oriented programming language terminology.

  • Containers

Containers boot up from images. Think of them as instances of a class.

  • Tags

You can think of this as versions

  • Container ID or Image ID

Unique ID assigned to every container or image respectively

Usage

  • A Dockerfile can be used to build docker images

  • To list all images available on your system

docker images
  • To list all containers
docker ps -a
  • To pull images from the docker hub, for example for pulling the ubuntu image
docker pull ubuntu

By default the latest tag is pulled. Consider tag to be sort of like the version of the image you want.

  • To pull a specific tag, lets say the tag is 12.04
docker pull ubuntu:12.04
  • To delete an image
docker rmi ubuntu:latest

or

docker rmi IMAGE_ID
  • To create a container from an image and run it
docker run -it ID /bin/bash
  • i stands for interactive
  • t stands for pseudo tty. It is for specifying the path to the shell to be run.
  • ID can be an IMAGE-ID or a CONTAINER-ID obtained by listing all images, or containers respectively.

Note: Every time you use docker run using the IMAGE-ID, a new container is created based on the image

Extra bits: If the argument --rm is if passed, causes the container to automatically get deleted after exiting the shell.

  • To exit a container
exit
  • To start a container
docker start CONTAINER_ID
  • To stop a container
docker stop CONTAINER_ID
  • To delete a container
docker rm CONTAINER_ID
  • To build your own docker image using a Dockerfile. Create a Dockerfile and put it in a folder, and execute the following command
docker build -t username/image_name:image_tag .

username - as on hub.docker.com image_name - the name of the image you want to build image_tag - the tag you want to assign to the image

  • To delete dangling images (unused images present on your system)
docker rmi $(docker images -f dangling=true -q)
  • Save an existing docker container as an image
docker commit CONTAINER_ID IMAGE_NAME:IMAGE_TAG
  • Saving an image as a tar file
docker save IMAGE_ID -o file_name.tar