Working with Docker images

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Introduction

Docker images are used to create Docker containers.

Docker images are the build component of Docker.

Picture credit : Understand the architecture

We create Docker containers using [base] images. An image can be basic, with nothing but the operating-system fundamentals, or it can consist of a sophisticated pre-built application stack ready for launch.

When we build images with docker, each action taken (i.e. a command executed such as apt-get install) forms a new layer on top of the previous one. These base images then can be used to create new containers.

Docker registries hold images.

These are public or private stores from which we can upload or download images.

The public Docker registry is provided with the Docker Hub.

What happens to an image at "docker run"?

We use "docker run" command from client to tell the Docker daemon to run a container, for example:

$ docker run -it ubuntu:latest /bin/bash

Basically, it's a container "launch" command.

At docker, a Docker client is launched and at run subcommand, a new container will be launched.

The new container will be built from ubuntu base image with "latest" tag.

Here i: interactive, : terminal.

As described in the official document, here are the things happening under the hood:

Pulls the ubuntu image:
Docker checks for the presence of the ubuntu image and, if it doesn't exist locally on the host, then Docker downloads it from Docker Hub.

If the image already exists, then Docker uses it for the new container.
Creates a new container:
Once Docker has the image, it uses it to create a container.
Allocates a filesystem and mounts a read-write layer:
The container is created in the file system and a read-write layer is added to the image.
Allocates a network and sets up an IP address:
Creates a network interface that allows the Docker container to talk to the local host.
Executes a process that we specify:
Runs our application.

Search images - docker search

docker search searches registry for image.

$ docker search ubuntu
NAME                              DESCRIPTION                                     STARS     OFFICIAL   AUTOMATED
ubuntu                            Ubuntu is a Debian-based Linux operating s...   4416      [OK]       
ubuntu-upstart                    Upstart is an event-based replacement for ...   65        [OK]  
...

Download images - docker pull

docker pull pulls an image from registry to local machine.

$ docker pull ubuntu
latest: Pulling from ubuntu
20ee58809289: Pull complete 
f905badeb558: Pull complete 
119df6bf2a3a: Pull complete 
94d6eea646bc: Pull complete 
bb4eabee84bf: Pull complete 
Digest: sha256:85af8b61adffea165e84e47e0034923ec237754a208501fce5dbeecbb197062c
Status: Downloaded newer image for ubuntu:latest

Docker images can consist of multiple layers.

In the example above, the image consists of five layers (20ee58809289,...,bb4eabee84bf).

We can use a tag to specify what to download. For example, 'latest' for tag:

$ docker pull ubuntu:latest

Listing images - docker images

To list the images on the host:

docker images
REPOSITORY                                 TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
ubuntu                                     latest              bb4eabee84bf        2 weeks ago         124.8 MB
ubuntu                                     16.04               bb4eabee84bf        2 weeks ago         124.8 MB
centos                                     7                   2a332da70fd1        9 weeks ago         196.8 MB
ubuntu                                     trusty              9bc953763843        10 weeks ago        188 MB
debian                                     latest              cea663c8c811        10 weeks ago        125.1 MB
centos                                     latest              ce20c473cd8a        9 months ago        172.3 MB

Running container - docker run

Now we want to launch a container:

$ docker run -it ubuntu:latest /bin/bash

We're logged in as a root:

root@859d4a27d4c8:/# whoami
root

Check the OS:

root@859d4a27d4c8:/# cat /etc/*release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
...

Let's check what processes are currently running:

root@859d4a27d4c8:/# top
...
  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
    1 root      20   0   18232   2032   1556 S   0.0  0.1   0:00.09 bash                                                        
   14 root      20   0   36628   1704   1272 R   0.0  0.0   0:00.02 top

As we can see there are only two processes, and they are isolated ones from the host processes.

Getting out of a container without stopping it

"Ctrl + P + Q" will do the trick:

root@859d4a27d4c8:/#

k@laptop:~$

From the PID, we can check the "top" on the host is using different space from the "top" on the docker container:

k@laptop:~$ pa aux | grep top
root     14512  0.0  0.0  36628  1704 pts/17   S+   22:01   0:00 top

Now, we may want to get back to our container again via "attach". To do that, we need to know the "CONTAINER ID":

k@laptop:~$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS              NAMES
859d4a27d4c8        ubuntu:latest       "/bin/bash"         5 minutes ago      Up 5 minutes   cranky_swartz

Let's do attach:

k@laptop:~$ docker attach 859d4a27d4c8
root@859d4a27d4c8:/#

Stops a container

We can stop the container and get out of it via "Ctrl + D":

root@859d4a27d4c8:/# exit

k@laptop:~$

If we issue "docker ps" command again, we see the container is not running anymore:

k@laptop:~$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS              NAMES
k@laptop:~$

We can list all containers including old ones that stopped running using "docker ps -a":

k@laptop:~$ docker ps -a
CONTAINER ID        IMAGE                                             COMMAND             CREATED             STATUS                     PORTS               NAMES
859d4a27d4c8        ubuntu:latest                                     "/bin/bash"         31 minutes ago      Exited (0) 4 minutes ago                       cranky_swartz        
...

Running a container in a background

To run a container in background, we use "docker start" command:

k@laptop:~$ docker start cranky_swartz
cranky_swartz
k@laptop:~$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
859d4a27d4c8        ubuntu:latest       "/bin/bash"         36 minutes ago      Up 4 seconds                            cranky_swartz

Check what processes are running inside a container

We can check what processes are running inside a container using "docker top":

k@laptop:~$ docker top cranky_swartz
UID                 PID                 PPID                C                   STIME               TTY                 TIME                CMD
root                15680               1559                0                   22:33               pts/17              00:00:00            /bin/bash

Stop a container running in background

To stop a container running in background, we use "docker stop" command:

k@laptop:~$ docker stop cranky_swartz

What's in the system

What's in our system related to the Docker containers?

root@laptop:/# tree /var/lib/docker/containers
|-- 859d4a27d4c883db39e68590f1d1d2c340f8775a94fe721eba85111d3b79c1fe
│   |-- 859d4a27d4c883db39e68590f1d1d2c340f8775a94fe721eba85111d3b79c1fe-json.log
│   |-- config.json
│   |-- hostconfig.json
│   |-- hostname
│   |-- hosts
│   |-- resolv.conf
│   |-- resolv.conf.hash

Note that there are no binary images for the container which makes docker consumes much less space compare to the other virtual tools.

Detach mode run

We can run a container in a "detach" mode, and later we can attach to it:

Note we need to use "-it" so that we can do something with the container after attaching. Also, we used "--name" to give our own name to the container.

k@laptop:~$ docker run -d -it --name=yaong ubuntu:16.04 /bin/bash
6bbba8e00d68a9b9c38bd7fdbd807dae01b9329d3b5ecd7ad2918305743bf5ea

Ok, it's started, and we can check it:

k@laptop:~$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS              PORTS               NAMES
6bbba8e00d68        ubuntu:16.04        "/bin/bash"         About a minute ago   Up About a minute                       yaong

To stop it:

k@laptop:~$ docker stop 6bbba8e00d68
6bbba8e00d68

k@laptop:~$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
k@laptop:~$