Docker - Image

This page is about the container image in Docker.

OCI is the standardized container format used by Docker

Docker stores downloaded images on the Docker host at the Docker Root Dir location

sudo ls /var/lib/docker/image/aufs

distribution       imagedb            layerdb            repositories.json

imageId
# or
[user/]image:tag
# or where tag default to latest (ie same repository:latest)
[user/]repository  

where:

• tag
• user identify user images. The user that created the image. For example the image training/sinatra has been created by the user training.

docker images

REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
docker-whale        latest              1351cae1fdfb        About a minute ago   275.1 MB
ubuntu              latest              f753707788c5        7 weeks ago          127.2 MB
hello-world         latest              c54a2cc56cbb        5 months ago         1.848 kB
docker/whalesay     latest              6b362a9f73eb        18 months ago        247 MB
training/webapp     latest              6fae60ef3446        19 months ago        348.8 MB

where

• repository is what repository they came from, for example ubuntu.
• tag are the tags for each image, for example 14.04.
• Image id: The image ID of each image.

name[:tag]

where:

• name
• tag

[user/]name

A repository potentially holds multiple variants of an image

In the case of the ubuntu image, there is multiple variants covering Ubuntu 10.04, 12.04, 12.10, 13.04, 13.10 and 14.04. Each variant is identified by a tag and you can refer to a tagged image like so:

ubuntu:14.04

A base image is a minimal linux image where you start to build more complicated image. See Docker - dockerfile

Example:

• Linux Container base image
• Windows Container base image

docker rmi -f (name or id)

where:

• name: name of the image
• id: id of the image

See also: Docker - Clean (Removing Image and Container)

Visualization of the image and their different layer:

• https://imagelayers.io
• https://github.com/justone/dockviz

Docker will automatically download any image you use that isn’t already present on the Docker host when you try to run it. If you want to pre-load an image you can download it using the docker pull

When doing a pull, you can see that each layer of the image has been pulled down

docker pull centos

Using default tag: latest
latest: Pulling from library/centos
f1b10cd84249: Pull complete
c852f6d61e65: Pull complete
7322fbe74aa5: Pull complete
Digest: sha256:90305c9112250c7e3746425477f1c4ef112b03b4abe78c612e092037bfecc3b7
Status: Downloaded newer image for centos:latest

To create an image, you can

• use a Dockerfile to specify instructions to (create|build) an image.
• create a commit from an existing container

docker commit containerName imageName

You can update a container created from an image and commit the results to an image.

You can commit the changes made to an image

docker commit -m "Added json gem" -a "Kate Smith"  0b2616b0e5a8 ouruser/sinatra:v2

where:

• -m: commit message
• -a: commit author
• 0b2616b0e5a8: the container ID
• ouruser/sinatra:v2: the target image

https://hub.docker.com/

# docker search searchTerm
docker search oracle

NAME                             DESCRIPTION                                     STARS     OFFICIAL   AUTOMATED
wnameless/oracle-xe-11g          Oracle Express 11g R2 on Ubuntu 16.04 LTS       352                  [OK]
oraclelinux                      Oracle Linux is an open-source operating s...   260       [OK]
alexeiled/docker-oracle-xe-11g   This is a working (hopefully) Oracle XE 11...   185                  [OK]
sath89/oracle-12c                Oracle Standard Edition 12c Release 1 with...   78                   [OK]
sath89/oracle-xe-11g             Oracle xe 11g with database files mount su...   76                   [OK]

where:

• OFFICIAL means that it comes from an official repository
• AUTOMATED means that the build is automated.

A container is a running instance of an image that you create with a run command.

See Docker - Containers

docker save [OPTIONS] IMAGE [IMAGE...]

To see the property of the image such as the entrypoint, you can use the inspect command

• Build your own images