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Saturday, 9 July 2022

How to Create Docker Image from scratch?

 

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Docker Commands:Interview Question


Docker Image

A Docker image is a file used to execute code in a Docker container. Docker images act as a set of instructions to build a Docker container, like a template. Docker images also act as the starting point when using Docker. An image is comparable to a snapshot in virtual machine (VM) environments.

Docker is used to create, run and deploy applications in containers. A Docker image contains application code, libraries, tools, dependencies and other files needed to make an application run. When a user runs an image, it can become one or many instances of a container.

Docker images have multiple layers, each one originates from the previous layer but is different from it. The layers speed up Docker builds while increasing reusability and decreasing disk use. Image layers are also read-only files. Once a container is created, a writable layer is added on top of the unchangeable images, allowing a user to make changes.

HOW TO CREATE DOCKER IMAGE?

Create a new directory hellodocker.

In that directory, create a new text file Dockerfile. Use the following contents:

FROM ubuntu:latest

CMD ["/bin/echo", "hello world"]

This image uses ubuntu as the base image. CMD command defines the command that needs to run. It provides a different entry point of /bin/echo and gives the argument “hello world”.

Build the image using the command:

  docker image build . -t helloworld

. in this command is the context for the command docker image build. -t adds a tag to the image.

The following output is shown:

Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM ubuntu:latest
latest: Pulling from library/ubuntu
9fb6c798fa41: Pull complete
3b61febd4aef: Pull complete
9d99b9777eb0: Pull complete
d010c8cf75d7: Pull complete
7fac07fb303e: Pull complete
Digest: sha256:31371c117d65387be2640b8254464102c36c4e23d2abe1f6f4667e47716483f1
Status: Downloaded newer image for ubuntu:latest
 ---> 2d696327ab2e
Step 2/2 : CMD /bin/echo hello world
 ---> Running in 9356a508590c
 ---> e61f88f3a0f7
Removing intermediate container 9356a508590c
Successfully built e61f88f3a0f7
Successfully tagged helloworld:latest

List the images available using docker image ls:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          latest              e61f88f3a0f7        3 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB

Other images may be shown as well but we are interested in these two images for now.

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Run the container using the command:

docker container run helloworld

to see the output:

hello world

If you do not see the expected output, check your Dockerfile that the content exactly matches as shown above. Build the image again and now run it.

Change the base image from ubuntu to busybox in Dockerfile. Build the image again:

docker image build -t helloworld:2 .

and view the images using docker image ls command:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          2                   7fbedda27c66        3 seconds ago       1.13MB
helloworld          latest              e61f88f3a0f7        5 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB
busybox             latest              54511612f1c4        9 days ago          1.13MB

helloworld:2 is the format that allows to specify the image name and assign a tag/version to it separated by :.



15. docker inspect <Container Id> -> It will give complete information about Container.

16. docker image prune –a -> It will delete images which doesn’t have even a Single Container.

17. docker run --rm <Image-Name> -> Create a docker container and auto remove on exit



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Check below link for question and answers with code:

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