Docker

A two part shell/management layer for building and running virtual linux containers, based on LXC.

A tool for deploying and running applications. Docker provides a way to run an application securely isolated in a container in a way that is platform agnostic.

Docker Elements

Image

Read-only template for a docker container.

  • An immutable file that's essentially a snapshot of a container. Images are created with the build command, and they'll produce a container when started with run.
  • Uses a union file system (UFS) to 'layer' file system branches on top of each other. Every time a change is made to a Docker image, a new layer is created.
  • Docker images are built from a set a steps called instructions. These instructions can be built either by executing commands manually or automatically through Dockerfiles.
  • As more layers (tools, applications, etc.) are added on top of the base, new images can be formed by committing these changes – like a version control system!
  • Images are stored in a Docker registry such as registry.hub.docker.com.
  • Because they can become quite large, images are designed to be composed of layers of other images, allowing a miminal amount of data to be sent when transferring images over the network.

Layers

layers

layers

Commands

command description

Search

docker search [image_name]
docker pull [image_name]

List all images on your system

docker images

Build an image from a dockerfile

docker build -t NAME_OF_IMAGE [directory where Dockerfile lives]

Commit an image

sudo docker commit [container ID] IMAGE_NAME

Remove

docker rmi -f IMAGE_ID

Container

A Linux Container, (sort of) like a directory, it holds everything needed for an app to run.

containers

  • Docker containers are essentially directories that can be packed (e.g. tar-archived), then shared and run on other hosts. The only dependency is having docker installed on the hosts.

  • Docker containers allow:

    • Application portability
    • Process isolation
    • Preventing access beyond the container's own filesystem
    • Lightweight, esp. relative to VMs

  • When everything is self-contained and the risk of system-level changes are eliminated, the container becomes immune to external exposures which could put it out of order (i.e. 'dependency hell').

  • NB: docker depends on a single process to run. When that process stops, the container stops.

  • If an image is a class, then a container is an instance of a class.

Commands

List

docker ps     # those running
docker ps -l  # both running & dormant

Create (either from an existing image or creating a new one)

docker run IMAGE_NAME COMMAND_TO_RUN
docker run my_image echo 'hello'

Running

docker run CONTAINER_ID
docker run IMAGE_NAME COMMAND_TO_RUN

Start an interactive shell within your container

docker run -it IMAGE_NAME /bin/sh

Forward port on the host to a port on the container

docker run --publish 3000:3000 IMAGE_NAME COMMAND_TO_RUN

Stopping a container

docker stop CONTAINER_ID

Removing a container

docker rm CONTAINER_ID

Attaching yourself to a container (your console will run commands within the container itself)

docker attach CONTAINER_ID

Detach the current container: type ^ + P followed by ^ + Q

Mounting

mounting volume

Installations

CentOS

https://docs.docker.com/engine/installation/linux/centos/

Update Yum

sudo yum update

Add the yum repo

sudo tee /etc/yum.repos.d/docker.repo <<-'EOF'
[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/
enabled=1
gpgcheck=1
gpgkey=https://yum.dockerproject.org/gpg
EOF

Install docker image

sudo yum install docker-engine

Start the daemon

sudo service docker start

Verify it worked

sudo docker run hello-world

Create docker group and add your user

sudo usermod -aG docker your_username

Start docker daemon at boot

sudo chkconfig docker on

Helpful Visuals

docker build and run

Docker Tools
Docker Engine runs on Linux to create the operating environment for your distributed applications.
Docker Machine automate Docker provisioning
Docker Toolbox an installer to quickly and easily install and setup a Docker environment on your computer
Kitematic build and run containers through a GUI

NOTE: Docker Machine deprecates Boot2Docker

References

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