Docker Basics

Basic Docker and Docker Compose concepts explained in a public-safe way.

Docker runs applications in containers. Containers are isolated environments that include an application and the things it needs to run.

What is Docker?

Docker is a tool for running applications in containers.

A container is not the same as a full virtual machine. It shares the host system kernel, but runs the application in an isolated environment.

Common uses:

web applications
databases
test environments
self-hosted services
development tools
background workers

What is an image?

An image is a template used to create a container.

Example:

nginx image
postgres image
redis image
application image

Pull an image:

docker pull IMAGE_NAME

Example:

docker pull nginx

What is a container?

A container is a running instance of an image.

Show running containers:

docker ps

Show all containers, including stopped ones:

docker ps -a

Run a simple container

Example:

docker run hello-world

Run nginx in the background:

docker run -d --name web-test -p 8080:80 nginx

This means:

-d = run in background
--name = container name
-p 8080:80 = host port 8080 maps to container port 80
nginx = image name

Test:

curl -I http://localhost:8080

Stop and remove:

docker stop web-test
docker rm web-test

Docker Compose

Docker Compose uses a YAML file to define one or more containers.

Common file name:

docker-compose.yml

or:

compose.yml

Start services:

docker compose up -d

Stop services:

docker compose down

Show logs:

docker compose logs

Follow logs:

docker compose logs -f

Simple Compose example

services:
  web:
    image: nginx:latest
    container_name: web-test
    ports:
      - "8080:80"
    restart: unless-stopped

Start:

docker compose up -d

Check:

docker ps
curl -I http://localhost:8080

Stop:

docker compose down

Ports

Ports expose a service from a container to the host.

Example:

ports:
  - "8080:80"

Meaning:

host port 8080 → container port 80

Then access:

http://localhost:8080

Volumes

Volumes store data outside the container.

This matters because containers can be deleted and recreated.

Example:

volumes:
  - ./data:/data

Meaning:

./data on host → /data inside container

Use volumes for:

database data
uploads
configuration
persistent application files

Environment variables

Environment variables pass settings into containers.

Example:

environment:
  - TZ=Europe/Prague
  - APP_ENV=production

Do not put real secrets in public examples.

Restart policy

Common restart policies:

no
always
unless-stopped
on-failure

Example:

restart: unless-stopped

Meaning:

restart container unless it was manually stopped

Container logs

Show logs for one container:

docker logs CONTAINER_NAME

Follow logs:

docker logs -f CONTAINER_NAME

With Compose:

docker compose logs -f SERVICE_NAME

Execute command inside container

docker exec -it CONTAINER_NAME bash

If bash is not available:

docker exec -it CONTAINER_NAME sh

Stop and remove containers

Stop:

docker stop CONTAINER_NAME

Remove:

docker rm CONTAINER_NAME

Stop and remove Compose stack:

docker compose down

Pull updates

Pull newer images:

docker compose pull

Recreate containers:

docker compose up -d

Check:

docker ps
docker compose logs

Docker networks

Docker creates networks so containers can talk to each other.

List networks:

docker network ls

Inspect network:

docker network inspect NETWORK_NAME

In Compose, services can usually reach each other by service name.

Example:

database service name: db
application can connect to: db

Docker volumes

List volumes:

docker volume ls

Inspect volume:

docker volume inspect VOLUME_NAME

Remove unused volumes:

docker volume prune

Be careful: removing volumes can delete persistent data.

Disk usage

Show Docker disk usage:

docker system df

Remove unused data:

docker system prune

Dangerous version:

docker system prune -a --volumes

This can remove images, containers, networks, and volumes that may contain data.

First command set

docker ps
docker ps -a
docker compose ps
docker compose logs -f
docker images
docker network ls
docker volume ls
docker system df

Common troubleshooting

Container is not running

docker ps -a
docker logs CONTAINER_NAME

With Compose:

docker compose ps
docker compose logs SERVICE_NAME

Port is already in use

Check listening ports:

ss -tulpn | grep :PORT

Then change the host port in Compose.

Example:

ports:
  - "8081:80"

Container restarts repeatedly

Check logs:

docker logs CONTAINER_NAME

or:

docker compose logs SERVICE_NAME

Look for:

permission errors
missing config
wrong environment variables
database connection problems
port conflicts

Dangerous actions

Be careful with:

docker rm
docker volume rm
docker volume prune
docker system prune -a --volumes
docker compose down -v
deleting bind mount folders
changing volume paths

These can delete data.