Docker Basics
Containers vs VMs, images, and core Docker commands (run, ps, stop, rm)
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Explanation
Docker packages your app and everything it needs (code, runtime, libraries, config) into a container — a portable, isolated process that runs the same everywhere.
Container vs Virtual Machine:
| | Container | Virtual Machine | |---|---|---| | What it includes | App + runtime + libs | Entire OS + app | | Startup time | ~milliseconds | ~minutes | | Disk size | ~MB | ~GB | | Isolation | Process-level (shared kernel) | Full OS isolation | | Use case | Microservices, CI/CD | Strong isolation needed |
Containers share the host OS kernel — they're not VMs. This makes them fast and lightweight.
Images vs Containers:
- Image — a read-only template (like a class). Stored on disk, versioned with tags.
- Container — a running instance of an image (like an object). Has its own writable layer.
Essential commands:
bash # Pull an image from Docker Hub docker pull python:3.12-slim # Run a container docker run python:3.12-slim python --version # -d: detached (background) -p: port map --name: friendly name docker run -d -p 8080:80 --name webserver nginx # List running containers docker ps docker ps -a # include stopped containers # Stop and remove docker stop webserver docker rm webserver docker stop $(docker ps -q) # stop all running containers # List images docker images # Remove an image docker rmi nginx
Running interactively:
bash # Start a bash shell inside a container docker run -it python:3.12-slim bash # Exec into a running container docker exec -it webserver bash
Port mapping — -p host_port:container_port: ``bash docker run -d -p 3000:80 nginx # Requests to localhost:3000 → forwarded to port 80 inside the container
Volumes — persisting data:
bash # Bind mount: maps a host directory into the container docker run -v /host/data:/app/data myapp # Named volume: Docker manages the storage location docker run -v mydata:/app/data myapp docker volume ls `` Without a volume, data written inside a container is lost when the container is removed.
Examples
Running your first container
`-d` runs in background; `-p 8080:80` maps host port 8080 to container port 80; `--name` gives a friendly reference
# Pull and run a Python container
docker pull python:3.12-slim
# Run a one-off command (container exits immediately after)
docker run python:3.12-slim python -c "print('Hello from Docker!')"
# Run a web server in the background
docker run -d --name myapp -p 8080:80 nginx
# curl localhost:8080 → nginx welcome page
# Check it's running
docker ps
# CONTAINER ID IMAGE STATUS PORTS NAMES
# a1b2c3d4e5f6 nginx Up 3 s 0.0.0.0:8080->80/tcp myapp
# View logs
docker logs myapp
# Stop and clean up
docker stop myapp
docker rm myappWorking with images and cleanup
`docker system prune` is the fastest way to reclaim disk space on a dev machine — be cautious on shared environments
# List all local images
docker images
# REPOSITORY TAG SIZE
# python 3.12-slim 131MB
# nginx latest 187MB
# Remove a specific image
docker rmi nginx
# See all containers (including stopped)
docker ps -a
# Remove a stopped container
docker rm container_id_or_name
# One-liner cleanup: remove all stopped containers
docker container prune
# Remove all unused images, containers, networks
docker system prune -aHow well did you understand this?
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