Introduction to Docker

What Docker is.

Docker runs applications in containers: isolated processes with their own filesystem view, dependencies, and configuration. Unlike a traditional virtual machine, a Linux container shares the host kernel. This often makes container deployment lighter than operating a separate guest OS for every service.

Images, containers, volumes.

• Image: the packaged filesystem and metadata from which containers are created.
• Container: a running or stopped instance of an image.
• Volume: persistent data stored outside a disposable container filesystem.
• Network: a way for containers to discover and communicate with permitted peers.

Launch a web server.

With Docker installed, this command runs an Nginx container and publishes its web port locally:

docker run --rm -d --name example-web -p 127.0.0.1:8080:80 nginx:stable

Visit http://127.0.0.1:8080 from the host. When you are finished, remove the container with docker stop example-web. Binding to 127.0.0.1 avoids exposing the example on every network interface.

Turn a command into configuration.

For repeatable deployments, put the service definition in compose.yaml:

services:
  web:
    image: nginx:stable
    ports:
      - "127.0.0.1:8080:80"
    restart: unless-stopped

Run docker compose up -d to start it and docker compose down to stop and remove the service container. A restart policy helps after crashes or daemon restarts; it does not replace monitoring or backups.

Design for maintenance.

• Pin known-good image versions rather than relying blindly on floating tags.
• Use named volumes or clearly managed directories for persistent data.
• Keep secrets out of repositories and public Compose examples.
• Publish only ports you need and restrict privileges.
• Back up both application data and the configuration required to restore it.
• Review upstream documentation before copying installation commands, which change over time.