Understanding Docker Internals: Images, Containers, and the Daemon

Modern software development demands consistency, portability, and scalability. One technology that revolutionized how we build and deploy applications is Docker.

But to truly understand Docker, we need to go deeper than just running.

In this article, we’ll clearly explain:

• What is inside a Docker Image?

• What is inside a Docker Container?

• What is the Docker Daemon?

• How all of them work together

Let’s break it down.

What Is a Docker Image?

A Docker Image is a read-only template used to create containers. Think of it as a blueprint or a packaged application snapshot.

What’s Inside a Docker Image?

A Docker image contains:

1. Base Operating System Layer

Usually a lightweight Linux distribution such as:

• Ubuntu

• Alpine

• Debian

It does not include a full operating system like a virtual machine. Instead, it shares the host system’s kernel.

2 . Application Code

Your actual project files:

• Python scripts

• Node.js applications

• React build files

• Java JAR files

3 . Dependencies

All required libraries and packages:

• Python packages (Flask, NumPy)

• Node modules

• System-level packages

4 . Runtime Environment

The required runtime version:

• Python 3.10

• Node 18

• OpenJDK

Important Concept: Image Layers

Docker images are built in layers.

Each instruction in a Dockerfile creates a new layer:

FROM python:3.10
COPY . .
RUN pip install -r requirements.txt

Each line = one layer.

This layered architecture makes Docker builds faster due to caching.

What Is a Docker Container?

A Docker Container is a running instance of a Docker image. If an image is a blueprint, a container is the actual running application.

What’s Inside a Container?

A container includes:

1. Everything from the Image

• Base OS layer

• Application code

• Dependencies

• Runtime

2.  A Writable Layer (Very Important)

Unlike images, containers add a writable layer on top.

When:

• Files are created

• Logs are written

• Data is modified

Those changes are stored in this writable layer.

This is why containers are:

• Temporary

• Replaceable

• Ephemeral

If you delete a container, the writable layer is removed (unless you use volumes).

3. Isolated Environment

Each container has:

• Its own file system

• Its own process space

• Its own network interface

But it shares the host machine’s kernel.

This is what makes containers lightweight compared to virtual machine

What Is the Docker Daemon?

The Docker Daemon (dockerd) is the background engine that powers Docker.

It runs as a system service and manages everything.

When you type a command like:

docker run nginx

You are interacting with the Docker Client, but the Docker Daemon actually does the work.

What Does the Docker Daemon Do?

The daemon is responsible for:

docker build : The daemon builds the image.

docker run : The daemon:

• Creates the container

• Allocates CPU and memory

• Sets up networking

• Starts the application process

docker pull nginx : The daemon contacts a Docker registry like Docker Hub, downloads the image, and stores it locally.

How Everything Works Together

Here’s what happens when you run:

docker run nginx

1. You send the command using the Docker Client

2. The Client communicates with the Docker Daemon

3. The Daemon checks if the image exists locally

4. If not, it pulls it from Docker Hub

5. The Daemon creates a container from the image

6. The container starts running

Simple flow:

User → Docker Client → Docker Daemon → Image → Container

Why This Architecture Matters

Understanding these three components helps you:

• Debug issues properly

• Optimize Docker builds

• Manage storage efficiently

• Design better production systems

• Prepare for tools like Kubernetes

Docker is not just about running commands — it’s about understanding how containerized systems work internally.