Docker Fundamentals & Multi-Stage Builds: A Concise Guide

Below is a consolidated “crash course” article on Docker images, containers, and multi-stage builds — drawing from all our discussions. You can revisit it anytime or share it with others as a knowledge guide.

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# **Docker Fundamentals & Multi-Stage Builds: A Concise Guide**

### **1. Images vs. Containers**

- **Docker Image**: A *read-only snapshot* of a filesystem that includes everything needed to run your app — operating system files, libraries, and application code.

- **Docker Container**: A *running instance* of an image that has a *writable layer* on top. You can think of containers as isolated processes on your system with their own filesystem, networking, etc.

When you run `docker run my-image`, Docker creates a container by taking the image’s read-only layers and stacking a new writable layer on top where your app can write logs or temporary files.

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### **2. Layered Filesystem & Build Process**

Docker’s build process is often described step-by-step:

1. **Docker reads** your `Dockerfile` from top to bottom.

2. For each instruction (`FROM`, `RUN`, `COPY`, etc.), Docker:

- Creates a *temporary container* from the most recent layer.

- Executes the instruction in that container (e.g., `RUN apt-get install …`).