Understand and publish images

3.1, Understanding of images

1. An image is a lightweight, executable, independent software package that contains all the content needed to run a certain software. We package applications and configurations into a complete, deliverable, and deployable operating environment, including code, libraries required for runtime, environment variables, and configuration files. This packaged operating environment is the image image file.
2. Only through image files can docker container instances be generated.

3.2, UnionFS (Union File System)

1. Union File System (UnionFS) is a layered, lightweight, high-performance file system. It is the basis of docker images, and supports file system modifications as a single submission to be superimposed layer by layer, and different directories can be mounted under the same virtual file system.
2. Images can be inherited through layering, and various specific application images can be made based on the basic image.

Characteristics of Union File System: Multiple file systems can be loaded at the same time, but from the outside, only one file system can be seen; Union loading will superimpose each layer of file system, so that the final file system will contain files and directories of all layers.

3.3, Image layering

When downloading an image, pay attention to the download log output, you can see that it is downloaded layer by layer:

# To view the image layer, you can use the command: docker image inspect image name
pi@yahboom:~$ docker image inspect mysql:latest
[
    {
        "Id": "sha256:5371f8c3b63eec64a33b35530be5212d6148e0940111b57b689b5ba1ffe808c8",
        .........
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:d6d4fc6aef875958d6186f85f03d88e6bb6484ab2dd56b30a79163baceff2f6d",
                "sha256:05c3b0b311a02bc56ca23105a76d16bc9b8c1d3e6eac808f4efb1a2e8350224b",
                "sha256:7b80f7f05642477ebc7d93de9539af27caab7c41a768db250fe3fe2b5506ca2c",
                "sha256:50e037faefab22cb1c75e60abb388b823e96a845650f3abd6d0a27e07a5a1d5e",
                "sha256:66040abb3f7201d2cc64531349a8225412db1029447a9431d59d999c941d56f6",
                "sha256:857162425652837a362aa5f1c3d4974cc83702728793de52ba48176d5367a89b",
                "sha256:7eebed3016f6b6ab68aa8e6be35f0689a3c18d331b7b542984a0050b859eaf26",
                "sha256:2fc4c142633d57d795edc0f3fd457f99a35fa611eab8b8c5d75c66e6eb729bc2",
                "sha256:7fde2d12d484f0c14dabd9ca845da0bcdaf60bd773a58ca2d73687473950e7fe",
                "sha256:9319848a00d38e15b754fa9dcd3b6e77ac8506850d32d8af493283131b9745a3",
                "sha256:5ff94d41f068ea5b52244393771471edb6a9a10f7a4ebafda9ef6629874a899b"
            ]
        },
        "Metadata": {
            "LastTagTime": "0001-01-01T00:00:00Z"
        }
    }
]

3.3.1, Layered Understanding

• All docker images start with a base image layer. When modifications or new content is added, a new image layer will be created on top of the current image layer.
• For a simple example, if a new image is created based on ubuntu 20.04, this is the first layer of the new image; if a python package is added to the image, a second image layer will be created on top of the base image layer; if a security patch is added, a third image layer will be created.
• Docker images are read-only. When the container is started, a new writable layer is loaded on top of the image! This layer is what we usually call the container layer, and everything below the container is called the image layer!

3.3.2, The benefits of using layered docker images

Resource sharing, for example, if multiple images are built from the same base image, then the host only needs to keep a base image on the disk, and only needs to load a base image in the memory, so that it can serve all containers, and each layer of the image can be shared.

3.4, Create and publish images

3.4.1, Create images

Method 1, submit an image from the container:

​x
# command
docker commit -m="committed description" -a="author" container id target image name to be created: [label name] [-m -a parameters can also be omitted]

# test
pi@yahboom:~$ docker ps -a
CONTAINER ID   IMAGE           COMMAND       CREATED       STATUS                   PORTS     NAMES
c54bf9efae47   ubuntu:latest   "/bin/bash"   3 hours ago   Up 24 minutes                      funny_hugle
3b9c01839579   hello-world     "/hello"      3 hours ago   Exited (0) 3 hours ago             jovial_brown
pi@yahboom:~$ docker commit c54bf9efae47 ubuntu:1.0
sha256:78ca7be949b6412f74ba12e8d16bd548aaa7c3fa25134326db3a67784f848f8f
pi@yahboom:~$ docker images   # Generated ubuntu:1.0 image
REPOSITORY                   TAG       IMAGE ID       CREATED         SIZE
ubuntu                       1.0       78ca7be949b6   5 seconds ago   69.2MB
yahboomtechnology/ros-foxy   3.4.0     49581aa78b6b   5 hours ago     24.3GB
yahboomtechnology/ros-foxy   3.3.9     cefb5ac2ca02   4 days ago      20.5GB
yahboomtechnology/ros-foxy   3.3.8     49996806c64a   4 days ago      20.5GB
yahboomtechnology/ros-foxy   3.3.7     8989b8860d17   5 days ago      17.1GB
yahboomtechnology/ros-foxy   3.3.6     326531363d6e   5 days ago      16.1GB
ubuntu                       latest    bab8ce5c00ca   6 weeks ago     69.2MB
hello-world                  latest    46331d942d63   13 months ago   9.14kB

Method 2: Dockerfile to create an image:

xxxxxxxxxx
# command
docker build -f dockerfile file path -t new image name: TAG . # The docker build command ends with a . to indicate the current directory
# test
docker build -f dockerfile-ros2 -t yahboomtechnology/ros-foxy:1.2 .

For more information about writing Dockerfile, please refer to: https://docs.docker.com/develop/develop-images/dockerfile_best-practices/

3.4.2. Publish images

A docker repository is a place where image files are stored. The largest public repository is dockerhub (https://hub.docker.com/), which stores a large number of images for users to download.

Steps to publish images to docker hub:

1. Address: https://hub.docker.com/, register an account first
2. Ensure that the account can be logged in normally

3. Use the tag command to modify the image name

The specification for publishing an image to docker hub is:

xxxxxxxxxx
docker push registered username/image name

For example, my registered username is: pengan88, so I need to modify the image name first

xxxxxxxxxx
# command:
docker tag image ID modified image name

# test
pi@yahboom:~$ docker images
REPOSITORY                   TAG       IMAGE ID       CREATED         SIZE
ubuntu                       1.0       78ca7be949b6   5 seconds ago   69.2MB
ubuntu                       latest    bab8ce5c00ca   6 weeks ago     69.2MB
hello-world                  latest    46331d942d63   13 months ago   9.14kB
pi@yahboom:~$ docker tag 78ca7be949b6 pengan88/ubuntu:1.0
pi@yahboom:~$ docker images
REPOSITORY                   TAG       IMAGE ID       CREATED          SIZE
pengan88/ubuntu              1.0       78ca7be949b6   23 minutes ago   69.2MB
ubuntu                       1.0       78ca7be949b6   23 minutes ago   69.2MB
ubuntu                       latest    bab8ce5c00ca   6 weeks ago      69.2MB
hello-world                  latest    46331d942d63   13 months ago    9.14kB

4. Log in to docker hub to publish the image:

xxxxxxxxxx
pi@yahboom:~$ docker login -u pengan88
Password:     # Enter the account and password registered with Docker Hub here
WARNING! Your password will be stored unencrypted in /home/jetson/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

Login Succeeded
pi@yahboom:~$ docker push pengan88/ubuntu:1.0
The push refers to repository [docker.io/pengan88/ubuntu]
ca774712d11b: Pushed
874b048c963a: Mounted from library/ubuntu
1.0: digest: sha256:6767d7949e1c2c2adffbc5d3c232499435b95080a25884657fae366ccb71394d size: 736

5. Visit docker hub to see that it has been successfully released