Docker Basics Training Guide

A practical beginner-friendly guide to Docker, containers, images, volumes, networks, Compose, backups, and everyday self-hosting commands.

What Docker is

Docker packages applications into isolated units called containers. A container bundles the application, runtime, dependencies, and startup configuration so it can run consistently across machines.

Docker is especially useful for self-hosting because it lets you run apps like Firefly III, databases, importers, dashboards, and helper services without installing all of their dependencies directly onto the host operating system.

The core mental model

Image  --run-->  Container
  |                 |
  |                 +-- ports: host:container, e.g. 8080:80
  |                 +-- volumes: persistent data storage
  |                 +-- networks: private communication between containers
  |
  +-- downloaded from a registry or built from a Dockerfile

First commands

docker run hello-world

docker run -d \
  --name my-nginx \
  -p 8080:80 \
  nginx:latest

Open http://localhost:8080 to see the web server.

Stop and remove it:

docker stop my-nginx
docker rm my-nginx

Common daily commands

Images and tags

Images usually look like this:

nginx:latest
mariadb:10.11
fireflyiii/core:latest
postgres:16-alpine

latest is convenient but can change unexpectedly. For databases, prefer stable version tags like mariadb:10.11 or postgres:16-alpine.

Containers

A container is a running instance of an image.

docker run -d --name demo redis:7-alpine

Useful docker run flags:

Ports

Port syntax is:

host_port:container_port

Example:

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

This means:

Browser -> localhost:8080 -> container port 80

Use this when you want local-only access:

ports:
  - "127.0.0.1:8080:80"

Volumes and persistent data

Containers are disposable. Volumes are where data survives.

Named volume:

docker volume create mydata
docker run -d --name db -v mydata:/var/lib/mysql mariadb:10.11

Bind mount:

docker run -d --name app -v /home/user/config:/config some-image

:::note Deleting a volume can delete your app/database data. Be careful with docker volume rm and especially docker compose down -v. :::

Networks

Docker Compose creates a private network for the services in a project. Containers can talk to each other using service names as hostnames.

In your Firefly III setup:

DB_HOST: db
DB_PORT: "3306"

db works because it is the Compose service name.

Docker Compose

Docker Compose defines multi-container applications in YAML.

Minimal example:

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

Daily Compose commands:

docker compose up -d
docker compose ps
docker compose logs -f
docker compose logs -f importer
docker compose restart importer
docker compose pull
docker compose up -d
docker compose down

Avoid this unless you intentionally want to delete volumes:

docker compose down -v

Environment variables and secrets

Inline variables:

services:
  app:
    image: example/app
    environment:
      TZ: "Australia/Melbourne"
      APP_URL: "http://localhost:8080"

Secrets in .env:

DB_PASSWORD='a-strong-password'

services:
  app:
    env_file:
      - .env

Never commit real .env files to a public repository.

Logs and debugging

Start here:

docker compose ps
docker compose logs -f SERVICE

Enter a container:

docker exec -it firefly-iii sh

Check environment variables:

docker exec firefly-importer env

Common problems:

Updates

cd ~/firefly-iii
docker compose pull
docker compose up -d

Backups

For databases, prefer application/database-level exports when available. For a raw named-volume backup:

docker run --rm \
  -v firefly-iii_firefly_iii_db:/volume \
  -v "$PWD":/backup \
  alpine \
  tar czf /backup/firefly-db-volume-backup.tar.gz -C /volume .

Security basics

• Do not expose admin apps directly to the public internet unless you know what you are doing.
• Prefer LAN-only access, VPN, Tailscale, WireGuard, or a carefully configured reverse proxy.
• Use strong unique passwords.
• Keep images updated.
• Use official or reputable images.
• Pin database image versions.
• Store secrets in .env, not public compose files.

Your Firefly III stack

Useful commands:

cd ~/firefly-iii

docker compose ps

docker compose logs -f firefly-iii

docker compose logs -f importer

docker compose restart importer

Rule of thumb

If you understand images, containers, ports, volumes, networks, and Compose, you understand most everyday Docker use.