How to Create a Docker Container from an Image

Creating a Docker container from an image is one of the fundamental steps to getting started with containerization. This article will guide you through the process of creating a Docker container from an existing image, explain the commands involved, and offer practical tips for managing containers. Whether you’re new to Docker or looking to refine your skills, this step-by-step guide will give you the knowledge to confidently create and manage Docker containers.

What is a Docker Image?

A Docker image is a lightweight, standalone, and executable software package that includes everything needed to run an application: code, runtime, libraries, environment variables, and configurations. Think of it as a blueprint for a Docker container. When you create a container, you're creating an isolated environment that runs based on this blueprint.

Key Differences Between Docker Images and Containers

• Docker Image: An immutable, read-only template with the application and dependencies.
• Docker Container: A runtime instance of a Docker image, with read/write layers.

In simpler terms, an image is the "recipe," and a container is the "meal" created from that recipe.

Steps to Create a Docker Container from an Image

Step 1: Install Docker

If you haven't installed Docker yet, download it from Docker's official site and follow the installation instructions for your operating system.

Once Docker is installed, open your terminal or command prompt and verify the installation by typing:

docker --version        

This command should display the Docker version, indicating that Docker is successfully installed.

Step 2: Pull an Image from Docker Hub

To create a container, you need an image. Docker Hub is a public registry where you can find pre-built images of various applications and OS environments. Let’s start by pulling an image from Docker Hub.

For example, to pull an nginx image (a popular web server), you’d use the command:

docker pull nginx        

This command downloads the latest version of the nginx image from Docker Hub to your local machine. You can specify a specific version by adding :tag after the image name, like nginx:1.21.6.

Step 3: Verify the Downloaded Image

To check if the image was successfully downloaded, list all images on your system:

docker images        

This command will show all available images along with their REPOSITORY, TAG, IMAGE ID, CREATED, and SIZE.

Step 4: Run the Docker Container from the Image

Now that you have the image, it’s time to create and run a container. The docker run command is used to start a container based on the image you pulled.

docker run -d --name my-nginx-container -p 8080:80 nginx        

• -d: Runs the container in detached mode (in the background).
• --name my-nginx-container: Assigns a name to the container, making it easier to reference.
• -p 8080:80: Maps port 8080 on your host to port 80 in the container, so you can access nginx at http://localhost:8080.
• nginx: The name of the image to run.

This command creates a container named my-nginx-container and runs nginx in the background, accessible via http://localhost:8080.

Step 5: Check Running Containers

To see the list of active containers, use:

docker ps        

This command will display all running containers, along with details like CONTAINER ID, IMAGE, COMMAND, CREATED, STATUS, PORTS, and NAMES.

To view all containers, including stopped ones, use:

docker ps -a        

Step 6: Interact with the Running Container

You can execute commands inside a running container using the docker exec command. For instance, to open a shell inside your nginx container:

docker exec -it my-nginx-container /bin/bash        

Here:

• -it: Allows interactive mode with a TTY attached.
• /bin/bash: The command to open a bash shell.

Once inside, you can interact with the container's file system or check logs, configuration files, etc.

Step 7: Stop and Remove the Container

To stop a running container, use:

docker stop my-nginx-container        

To remove a container after stopping it:

docker rm my-nginx-container        

You can also combine both actions with docker rm -f, which forcefully stops and removes the container:

docker rm -f my-nginx-container        

Step 8: Clean Up Images (Optional)

Once you’re done with an image, you can remove it to free up space using:

docker rmi nginx        

This will delete the nginx image if no containers are currently using it.

Practical Tips for Managing Docker Containers

1. Use Named Containers: Assign names to containers with the --name flag to make managing multiple containers easier.
2. Map Ports Carefully: Use the -p flag to expose the correct ports for your application.
3. Environment Variables: You can pass environment variables into containers with the -e flag in the docker run command, like -e ENV_VAR=value.
4. Mount Volumes for Persistence: Use -v to mount a local directory as a volume to retain data outside of the container lifecycle, such as -v /host/path:/container/path.
5. Check Logs: Monitor container activity by checking logs with docker logs <container_name>.