What is Docker? Docker Container: A Deep Dive into Docker

Docker is an open-source platform that automates the deployment of applications inside lightweight, portable units called containers. A container packages an application's code together with everything it needs to run — libraries, dependencies, environment variables, and configuration files — so it behaves the same way regardless of where it's deployed.

Images vs Containers

Two terms are central to understanding Docker:

• Docker Image: A read-only template that defines what goes into a container — the base operating system layer, application code, dependencies, and startup instructions. Images are built from a Dockerfile.
• Docker Container: A running instance of an image. You can start, stop, move, or delete a container, and you can create many containers from the same image.

How Docker Containers Work

Docker containers rely on operating system-level virtualization features built into the Linux kernel:

• Namespaces: Isolate a container's view of the system — process IDs, network interfaces, mount points, and hostnames — so it appears to have its own environment.
• Control Groups (cgroups): Limit and account for a container's use of CPU, memory, disk I/O, and network resources.
• Union File Systems: Allow images to be built in layers, where each instruction in a Dockerfile creates a new layer. Layers are cached and reused, making builds and image transfers efficient.

The Docker Architecture

A Simple Dockerfile Example

Here's a minimal Dockerfile for a Node.js application, showing how an image is defined layer by layer:

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

Container vs Virtual Machine