What is a Docker Image and How is It Different from a Container?

A Docker image is a small and easy-to-use software package. It has everything we need to run a program. This includes the code, runtime, libraries, and system tools. We can think of it as a snapshot of a filesystem. We use it to create Docker containers.

It is important to know the difference between a Docker image and a Docker container. The image is the template. We make containers from this template. Containers are the running versions of those images.

In this article, we will look at Docker images and how they are different from containers. We will talk about how Docker images are built. We will also learn how to create them using a Dockerfile. Then we will understand what a running container is and how it connects to images.

Next, we will show how to list and manage Docker images and containers. We will give a simple example of how to build and run a Docker image. Finally, we will answer some common questions about Docker images and containers.

  • What are Docker Images and How Do They Differ from Containers?
  • Understanding Docker Image Architecture
  • How to Create a Docker Image from a Dockerfile
  • What is a Running Container and How Does it Relate to Images?
  • How to List and Manage Docker Images and Containers
  • Practical Example: Building and Running a Docker Image
  • Frequently Asked Questions

Understanding Docker Image Architecture

We see that Docker images have a layered file system. This helps us store and share containerized applications in a smart way. Each layer shows a set of file changes or instructions from a Dockerfile. This way, we can reuse layers and save space.

Key Components of Docker Image Architecture:

  • Layers: Each image has many layers stacked on each other. We create these layers from the instructions in the Dockerfile. For example, one layer might install a package. Another layer might copy files into the image.

  • Union File System: Docker uses a union file system. This combines all layers into one view. Now, containers can access files from all layers without making copies.

  • Read-Only Layers: The layers below are fixed and read-only. When we create a container from an image, we add a thin writable layer on top. Here, the container can make changes.

  • Dockerfile: This text file has a list of commands to build the image. Important instructions include:

    • FROM: This tells which base image to use.
    • RUN: This runs commands while building the image.
    • COPY or ADD: This copies files to the image.
    • CMD or ENTRYPOINT: This sets the default command to run when the container starts.

Example Dockerfile:

# Use an official Python runtime as a parent image
FROM python:3.9-slim

# Set the working directory in the container
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install any needed packages specified in requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]

Image Metadata:

Every Docker image is saved in a registry. It has metadata that includes: - Image ID: This is a unique ID for the image. - Tags: These give version info for images. This helps us manage and deploy easily. - Size: This shows the total size of the image, including all layers.

Image Repositories:

We can store Docker images in a repository like Docker Hub. This gives us a central place to share images. Developers can pull images from the repository for use on their local machines or servers.

To learn more about Docker images and how they work, check this article on what are Docker images and how do they work.

How to Create a Docker Image from a Dockerfile

Creating a Docker image from a Dockerfile is a basic task. This helps us automate the image creation process. A Dockerfile is a text file. It has all the commands we need to build an image. To create a Docker image, we can follow these steps:

  1. Create a Dockerfile: First, we need to make a file called Dockerfile in our project folder. This file should not have any extension.

  2. Define the Base Image: We use the FROM instruction to choose the base image. For example, if we want to use Ubuntu as the base image, we write:

    FROM ubuntu:latest

  3. Install Dependencies: We use the RUN instruction to install any needed tools for our application. For example, to install curl, we write:

    RUN apt-get update && apt-get install -y curl

  4. Copy Application Files: We use the COPY instruction to move files from our local computer to the image:

    COPY . /app

  5. Set the Working Directory: We use the WORKDIR instruction to set the working folder inside the image:

    WORKDIR /app

  6. Expose Ports: If our application uses a specific port, we use the EXPOSE instruction:

    EXPOSE 8080

  7. Define the Entry Point: We use the CMD instruction to tell the image what command to run for our application:

    CMD ["python", "app.py"]

Here is an example of a full Dockerfile for a simple Python application:

# Use an official Python runtime as a parent image
FROM python:3.8-slim

# Set the working directory
WORKDIR /usr/src/app

# Copy the current directory contents into the container at /usr/src/app
COPY . .

# Install any needed packages specified in requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]

  1. Build the Docker Image: We go to the folder with our Dockerfile. Then we run this command to build the image:

    docker build -t my-python-app .

We can change my-python-app to any name we want for the image.

  1. Verify the Image Creation: We can list the Docker images we have by using this command:

    docker images

This shows us all the images, including the one we just made. For more information about Docker images and how they work, we can check What are Docker Images and How Do They Work?.

What is a Running Container and How Does it Relate to Images?

A running container is a version of a Docker image that is now active. A Docker image is a file that has all the parts needed to run an app. These parts include code, libraries, environment variables, and configuration files. A container is the active part of an app that comes from that image.

Key Differences between Containers and Images

  • State: An image does not change. It stays the same. A container is a running version of the image. It can change its state, like saving data to the filesystem.
  • Lifecycle: We can create, change, and share images. Containers can be started, stopped, paused, or removed.
  • Isolation: Containers run in separate environments. They share the host OS kernel but have their own processes and file systems.

Container Creation and Execution

We can create and run a container with the docker run command. For example:

docker run -d --name my_container my_image:latest

This command does these things: - -d: It runs the container in the background. - --name my_container: It gives a name to the container. - my_image:latest: It shows which Docker image to use.

Interaction with Running Containers

When a container is running, we can interact with it using different Docker commands:

  • List running containers:

    docker ps

  • Stop a running container:

    docker stop my_container

  • Access the container’s shell:

    docker exec -it my_container /bin/bash

Relationship with Images

  • Creation: We create containers from images. When we start a container, it makes a writable layer on top of the image layers.

  • Modifications: Changes in a running container do not change the original image. If we want to save changes, we can commit the container to a new image using:

    docker commit my_container new_image_name:tag

Knowing how Docker images and running containers relate is important for good container use and app deployment. If we want to learn more about Docker images, we can check out What are Docker Images and How Do They Work?.

How to List and Manage Docker Images and Containers

We can manage Docker images and containers using several Docker CLI commands. Here are the important commands for listing, removing, and managing Docker images and containers.

Listing Docker Images

To see all Docker images on our system, we use:

docker images

This command shows a list with these columns:

  • REPOSITORY: Name of the image.
  • TAG: Version of the image.
  • IMAGE ID: Unique ID for the image.
  • CREATED: When the image was made.
  • SIZE: Size of the image.

Listing Docker Containers

To see all running containers, we use:

docker ps

If we want to see all containers, including stopped ones, we use:

docker ps -a

The result will show:

  • CONTAINER ID: Unique ID for the container.
  • IMAGE: The image that made the container.
  • COMMAND: The command the container runs.
  • CREATED: Time since the container was made.
  • STATUS: Current state (running, exited, etc.).
  • PORTS: Open ports.
  • NAMES: Name of the container.

Removing Docker Images

To delete a specific Docker image, we use:

docker rmi <image_name_or_id>

For example:

docker rmi my_image:latest

To force delete an image, we use the -f flag:

docker rmi -f <image_name_or_id>

Removing Docker Containers

To delete a stopped container, we use:

docker rm <container_id>

To delete all stopped containers, we can combine commands:

docker container prune

Managing Docker Images and Containers

  • Tagging Images: To tag an image for easy identification:
docker tag <image_id> <repository>:<tag>
  • Building Images: To create an image from a Dockerfile:
docker build -t <image_name>:<tag> .
  • Running Containers: To run a container from an image:
docker run -d --name <container_name> <image_name>
  • Stopping Containers: To stop a running container:
docker stop <container_id_or_name>
  • Starting Containers: To start a stopped container:
docker start <container_id_or_name>

For more details on Docker images and containers, check this guide on Docker images and how they work.

Practical Example: Building and Running a Docker Image

We want to show how to build and run a Docker image. We will create a simple app using a Dockerfile. This example will help us understand how to make a Docker image from the beginning and run a container from that image.

Step 1: Create a Simple Application

Let us make a simple Node.js app. First, we create a folder for our project:

mkdir my-node-app
cd my-node-app

Next, we will create a file called app.js:

// app.js
const http = require('http');

const hostname = '0.0.0.0';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

Step 2: Create a Dockerfile

In the same folder, we create a file called Dockerfile with this content:

# Use the official Node.js image as a base
FROM node:14

# Set the working directory inside the container
WORKDIR /usr/src/app

# Copy package.json and install dependencies
COPY package*.json ./
RUN npm install

# Copy the application code
COPY . .

# Expose the port the app runs on
EXPOSE 3000

# Define the command to run the app
CMD ["node", "app.js"]

Step 3: Create a package.json File

We need a package.json file to manage our Node.js app’s dependencies. Create a file called package.json:

{
  "name": "my-node-app",
  "version": "1.0.0",
  "description": "A simple Node.js app",
  "main": "app.js",
  "scripts": {
    "start": "node app.js"
  },
  "dependencies": {}
}

Step 4: Build the Docker Image

Now we open our terminal and go to the project folder. We run this command to build the Docker image:

docker build -t my-node-app .

Step 5: Run the Docker Container

After we finish building the image, we can run it as a container:

docker run -p 3000:3000 my-node-app

Step 6: Access the Application

We open our web browser and go to http://localhost:3000. We should see “Hello World” in our browser. This means our Docker container runs good.

This practical example shows how to create a Docker image and run a container from that image. It shows how easy and powerful Docker can be for deploying apps. For more info on Docker images, we can check this article.

Frequently Asked Questions

1. What is a Docker image?

A Docker image is a small and standalone package. It contains everything we need to run a piece of software. This includes the code, runtime, libraries, and dependencies. Think of it as a snapshot of an application and its environment. This helps us to deploy software consistently on different platforms. For more details, check our article on what are Docker images and how do they work.

2. How do Docker images differ from containers?

Docker images are static files. They define what a container will run. Containers are the running parts of these images. So, we can say an image is like a blueprint. A container is the actual building made from that blueprint. To learn more about how Docker containers work, see our discussion on what is a Docker container and how does it operate.

3. Can I create a Docker image without a Dockerfile?

Yes, we can create a Docker image without a Dockerfile. We can use the docker commit command to save changes from a running container. But using a Dockerfile is better. It gives us a clear and repeatable way to build images. For help on creating Docker images well, see our section on how to create a Docker image from a Dockerfile.

4. How do I list all Docker images and containers on my system?

To list all Docker images, we use the command docker images. To see all running containers, we use docker ps. If we want to view all containers, even the stopped ones, we use docker ps -a. Managing our images and containers well is very important for using Docker. We can explore more about managing Docker images and containers in our article on how to list and manage Docker images and containers.

5. What are the benefits of using Docker images in development?

Using Docker images in development helps us keep things the same across different environments. It makes managing dependencies easier. It also allows us to deploy applications quickly. Docker images support microservices and container orchestration. This makes them very important for modern software development. For more insights into the benefits of Docker, check our article on what are the benefits of using Docker in development.