Docker Images

Docker images are the building blocks of Docker containers. An image is a read-only template that contains the instructions for creating a Docker container. It includes everything needed to run an application, including code, a runtime, libraries, environment variables, and config files.

Images are created by writing a Dockerfile, which is a script that specifies the environment and configuration of a Docker container. The Dockerfile contains instructions for installing software packages, setting environment variables, and running commands. Once the Dockerfile is complete, it can be built into an image using the docker build command.

Docker images are stored in a registry, such as Docker Hub, which is a central repository for Docker images. Docker Hub provides a public registry of images that are free to use, as well as private registries that require authentication to access.

To use a Docker image, you must first download it to your local machine using the docker pull command. Once the image is downloaded, it can be used to create a Docker container using the docker run command.

Example

Here is an example Dockerfile for a simple Node.js application:

FROM node:14

WORKDIR /app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE 3000

CMD [ "npm", "start" ]

This Dockerfile specifies that the image should be based on the official Node.js 14 image, set the working directory to /app, copy the package.json file, install dependencies using npm, copy the application code to the container, expose port 3000, and run the npm start command when the container starts.

Once the Dockerfile is created, it can be built into an image using the following command:

docker build -t my-node-app .

This will create an image with the tag my-node-app that can be used to create a Docker container.

Conclusion

Docker images are a fundamental concept in Docker. They are used to create Docker containers and define the environment and configuration of an application. Images are created by writing a Dockerfile and can be stored in a registry for easy distribution and sharing.

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