Docker Day 1: Grasp Core Concepts in 5 Minutes and Stop Treating Containers as VMs

1. Clarifying Three Core Concepts

Image – a read‑only blueprint, analogous to a class definition or a recipe.

Container – the instantiated, running object, analogous to an object instance or a cooked dish.

Registry – the storage for images, comparable to a Maven/Go‑mod repository or a marketplace.

One image can spawn multiple independent containers.

2. Containers vs. Virtual Machines

Virtual Machine: Full OS + Hypervisor → Strong isolation, slow start (minutes), high memory use
Container:      Shared host kernel → Weaker isolation (but sufficient), fast start (seconds), low resource use

Key point: A container is a process, not a virtual machine.

Practical example: Starting a local MySQL instance takes about 5 minutes and 2 GB of RAM on a VM, but only 3 seconds and 200 MB on a Docker container, a huge advantage during development.

3. Your First Commands

# Pull an image and run a container
docker run hello-world

# List local images
docker images

# Show running containers
docker ps

# Show all containers (including stopped)
docker ps -a

Running the hello-world image completes the full "pull → create → run" flow.

4. Core Command Set

# Pull an image
docker pull ubuntu:20.04

# Run a container interactively
docker run -it ubuntu:20.04 bash   # -i for interactive, -t allocates a TTY
# Exiting the shell stops the container (whether it is removed depends on options)

# Run a container in the background
docker run -d -p 8080:80 nginx   # -d daemon mode, -p hostPort:containerPort

# Stop / remove a container
docker stop <container_id>
docker rm <container_id>

# Exec into a running container
docker exec -it <container_id> bash

5. Official Docker Concept Diagram

┌─────────────────────────────────────────────┐
│               Docker Host                  │
│  ┌─────────────┐      ┌─────────────────┐ │
│  │   Images    │      │   Containers    │ │
│  │ (static)   │ new  │   (dynamic)     │ │
│  └─────────────┘ ────▶ └─────────────────┘ │
│          ▲                                 │
│          │ pull from                        │
│  ┌───────┴───────┐                         │
│  │   Registry    │ Docker Hub / private repo │
│  └───────────────┘                         │
└─────────────────────────────────────────────┘

6. Day 1 Summary

Containers are processes, not virtual machines.

Image → Container corresponds to Class → Instance.

Core commands to remember: run, ps, images, exec, stop, rm.