003-docker-containers.md

Chapter 3: Working with Docker Containers

Docker containers are lightweight, standalone, and executable packages that include everything needed to run a piece of software, including the code, runtime, system tools, libraries, and settings. In this chapter, we'll explore how to work with Docker containers effectively.

Running Your First Container

Let's start by running a simple container:

docker run hello-world

This command does the following:

1. Checks for the hello-world image locally
2. If not found, pulls the image from Docker Hub
3. Creates a container from the image
4. Runs the container, which prints a hello message
5. Exits the container

Basic Docker Commands

Here are some essential Docker commands for working with containers:

Listing Containers

To see all running containers:

docker ps

To see all containers (including stopped ones):

docker ps -a

Starting and Stopping Containers

To stop a running container:

docker stop <container_id_or_name>

To start a stopped container:

docker start <container_id_or_name>

To restart a container:

docker restart <container_id_or_name>

Removing Containers

To remove a stopped container:

docker rm <container_id_or_name>

To force remove a running container:

docker rm -f <container_id_or_name>

Running Containers in Different Modes

Detached Mode

Run a container in the background:

docker run -d <image_name>

Interactive Mode

Run a container and interact with it:

docker run -it <image_name> /bin/bash

Port Mapping

To map a container's port to the host:

docker run -p <host_port>:<container_port> <image_name>

Example:

docker run -d -p 80:80 nginx

Working with Container Logs

View container logs:

docker logs <container_id_or_name>

Follow container logs in real-time:

docker logs -f <container_id_or_name>

Executing Commands in Running Containers

To execute a command in a running container:

docker exec -it <container_id_or_name> <command>

Example:

docker exec -it my_container /bin/bash

Practical Example: Running an Apache Container

Let's run an Apache web server container:

1. Pull the image:

   docker pull httpd

2. Run the container:

   docker run -d --name my-apache -p 8080:80 httpd

3. Verify it's running:

   docker ps

4. Access the default page by opening a web browser and navigating to http://localhost:8080

5. Modify the default page:

   docker exec -it my-apache /bin/bash
   echo "<h1>Hello from my Apache container!</h1>" > /usr/local/apache2/htdocs/index.html
   exit

6. Refresh your browser to see the changes

Container Resource Management

Limiting Memory

Run a container with a memory limit:

docker run -d --memory=512m <image_name>

Limiting CPU

Run a container with CPU limit:

docker run -d --cpus=0.5 <image_name>

Container Networking

Listing Networks

docker network ls

Creating a Network

docker network create my_network

Connecting a Container to a Network

docker run -d --network my_network --name my_container <image_name>

Data Persistence with Volumes

Creating a Volume

docker volume create my_volume

Running a Container with a Volume

docker run -d -v my_volume:/path/in/container <image_name>

Container Health Checks

Docker provides built-in health checking capabilities. You can define a health check in your Dockerfile:

HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
  CMD curl -f http://localhost/ || exit 1

Cleaning Up

Remove all stopped containers:

docker container prune

Remove all unused resources (containers, networks, images):

docker system prune

Conclusion

Working with Docker containers involves a range of operations from basic running and stopping to more advanced topics like resource management and networking. As you become more comfortable with these operations, you'll be able to leverage Docker's full potential in your development and deployment workflows.

Remember, containers are designed to be ephemeral. Always store important data in volumes or use appropriate persistence mechanisms for your applications.