Docker Notes 1 - Beginner

Basic Concept

Docker can be seen as a simplified virtual machine (VM).
Since it doesn’t install a full operating system, it offers a smaller footprint and faster speed.

Image

An Image contains a lightweight runtime environment,
including its libraries and executables.

Images can be thought of as the .iso file for a Docker VM.
It can only be read, not executed directly.
If one wants to modify an image, they can only create a new image based on the existing one.

Container

When an image is used to create a running environment,
it becomes a container.

Like a VM,
the Docker container is isolated from the host environment.
Whatever is done within the container does not affect the host environment,
unless specific settings are configured.

For example,
We can expose a port of a Docker container,
while the host port remains closed.
However, we can also expose the host’s port if desired.

Repository

A repository is where images are stored.

It is similar to a Git repository:
There can be many repositories,
and each serves as a place to store the code of a project.
Similarly, Docker repositories are where images are stored.
Every image within the same repository shares the same name but has different tags.
Additionally, there are many different repositories for various images.

Registry

A registry is also a place to store images.

The difference between a registry and a repository is that a registry is a service where users can push or pull images to and from their local machines, similar to GitHub for Git.
The most famous one is Docker Hub.
Whereas a repository is a location to keep images with the same name but different tags.

Basic Usage of Docker

Install

It is very simple in Ubuntu:

sudo apt-get install docker.io

Pull Image

There are many images on Docker Hub that can be used.

If I want a clean Ubuntu environment,
I can pull an Ubuntu image to my local machine:

docker pull ubuntu

Or if I want to specify a tag:

docker pull ubuntu:14.04

Run the Image

We can echo Hello world using the image we just obtained:

docker run ubuntu /bin/echo 'Hello world'

This should print Hello world on the terminal.

What just happened is that the docker run command creates a temporary container,
and terminates itself after the echo command completes.

List Images at Local

docker images

It should list the image we just pulled to our local machine.

Create a Container

Once we have a container,
we have a running environment that can be modified by our actions.

Creating a container from an image is just like using an .iso to create a virtual machine.

We can create a container running Ubuntu with the image we pulled:

docker create -it ubuntu

We can also create a container with a name:

docker create -it --name CONTAINER_NAME ubuntu

i refers to ‘interactive’ (opens stdin of the container).
t refers to ‘TTY’ (allocates a pseudo-TTY so we can interact with it via a terminal).

Or if we want to create a container and run it:

docker run -itd ubuntu

or

docker run -itd --name CONTAINER_NAME ubuntu

d refers to ‘detach’ (runs the container in the background).

List Containers

docker ps -a

This should list all containers on the host machine.

And we can observe a difference in status between containers created with docker create and docker run:
Containers created with docker create are only created and not yet running,
so their status is created;
whereas containers created with docker run are both created and run,
so their status is up.

There is a container ID that can be used to run or terminate the container.

Run Containers

So,
if a container is created with docker create, it must be run before we can access it:

We can use the container ID to run the container:

docker start "CONTAINER_ID"

Or,
if the container was created with a name,
we can use that name to run it:

docker start "CONTAINER_NAME"

If the container’s status is exited,
it also needs to be started to run before we can access it.
Use

docker ps -a

to check the status first.

For containers created with docker run,
or those already started with docker start,
we can access them with docker exec:

docker exec -it "CONTAINER_ID" bash

bash is the command we want to run;
it can be replaced with other commands like echo or anything else.

We can also use the container’s name to access it:

docker exec -it "CONTAINER_NAME" bash

If bash is the command used,
we should find ourselves inside the container.
The user becomes root,
and we can start configuring settings or installing software within the container.

If we want to leave the container:

exit

The container remains running in the background after we exit it.

Stop Containers

This is quite similar to turning off a virtual machine.
Stopping a container only changes its status to exited;
it does not remove the container entirely.

docker stop "CONTAINER_ID"

Or,

docker stop "CONTAINER_NAME"

If we inspect it with,

docker ps -a

We will find that the container still exists,
but its status has changed to exited.

Export Container

Once a container is exported,
it can be moved to another host machine.

We can export a container as a .tar file.

For example,
export a container as exported.tar:

docker export "CONTAINER_ID" > exported.tar

Or,

docker export "CONTAINER_NAME" > exported.tar

Then we can move the .tar file to another machine.

Reference

Docker docs