Docker 3 - How to manage Docker images

Introduction

• Docker Images Management:
  • Docker images are large files stored on your PC.
  • Tools for Management:
    • Use Docker CLI and Docker Desktop to build, list, remove, and run images.
  • How It Works:
    • Management is done through the Docker client (docker command).
    • The client sends commands to the dockerd daemon, which executes them.

How to Build a Docker Image

• We use the docker build command to build Docker images.
• Let’s assume we use the Dockerfile definition from earlier to build an image.
• Here’s an example that shows the build command:

docker build -t temp-ubuntu .

• Here’s the output the build command generates:

Sending build context to Docker daemon  4.69MB
Step 1/8 : FROM ubuntu:18.04
 ---> a2a15febcdf3
Step 2/8 : RUN apt -y update && apt install -y wget nginx software-properties-common apt-transport-https && wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb && dpkg -i packages-microsoft-prod.deb && add-apt-repository universe && apt -y update && apt install -y dotnet-sdk-3.0
 ---> Using cache
 ---> feb452bac55a
Step 3/8 : CMD service nginx start
 ---> Using cache
 ---> ce3fd40bd13c
Step 4/8 : COPY ./default /etc/nginx/sites-available/default
 ---> 97ff0c042b03
Step 5/8 : WORKDIR /app
 ---> Running in 883f8dc5dcce
Removing intermediate container 883f8dc5dcce
 ---> 6e36758d40b1
Step 6/8 : COPY ./website/. .
 ---> bfe84cc406a4
Step 7/8 : EXPOSE 80:8080
 ---> Running in b611a87425f2
Removing intermediate container b611a87425f2
 ---> 209b54a9567f
Step 8/8 : ENTRYPOINT ["dotnet", "website.dll"]
 ---> Running in ea2efbc6c375
Removing intermediate container ea2efbc6c375
 ---> f982892ea056
Successfully built f982892ea056
Successfully tagged temp-ubuntu:latest

What is an image tag?

• Image Tag:

  • A text string used to version a Docker image.
  • Example: temp-ubuntu:latest where latest is the tag.

• Tagging Images:

  • Use the -t flag during image build to name and tag an image.
  • If no tag is specified, the image is automatically labeled with the latest tag.

• Multiple Tags:

  • An image can have multiple tags.
  • Conventionally, latest is used for the most recent version, along with a tag for the version number.
  • When releasing a new version, you can reassign the latest tag to point to the new image.

• Windows Base Container Images:

  • Microsoft doesn’t provide a latest tag for Windows base images.
  • You must specify a specific tag, such as ltsc2016, ltsc2019, or ltsc2022 for Windows Server Core images.

• Example with .NET Core:

  • Microsoft offers multiple .NET Core sample images, each tagged for different platforms like aspnetapp, wcfservice, and dotnetapp.
  • Tags help specify which sample or version of .NET Core the image is related to.

How to List Docker Images

• Listing Images:

  • Use the docker images command to list all images on your PC.
  • Here’s an example of the output:

docker images

• Output:

REPOSITORY          TAG                     IMAGE ID            CREATED                     SIZE
tmp-ubuntu          latest             f89469694960        14 minutes ago         1.69GB
tmp-ubuntu          version-1.0        f89469694960        14 minutes ago         1.69GB
ubuntu              18.04                   a2a15febcdf3        5 weeks ago            64.2MB

• Notice how the image is listed with its Name, Tag, and an Image ID.
• Recall that we can apply multiple labels to an image.
• The preceding output shows an example; even though the image names are different, we can see the IDs are the same.
• The image ID is a useful way to identify and manage images where the name or tag of an image might be ambiguous.

How to remove an image

• You can remove an image from the local docker registry with the docker rmi command.
• This is useful if you need to save space on the container host disk, because container image layers add up to the total space available.
• Specify the name or ID of the image to remove.
• This example removes the image for the sample web app using the image name:

docker rmi temp-ubuntu:version-1.0

• You can’t remove an image if a container is still using the image.
• The docker rmi command returns an error message, which lists the container that relies on the image.