Docker - Basic

Volumes - Making Data Survive

Here's something that surprises a lot of people the first time: anything a container writes to its own filesystem disappears the moment that container is removed. Containers are meant to be disposable, so their disk is disposable too, by design.

That's a problem the moment you have data you actually need to keep - a database's files, uploaded images, anything that matters. A volume is Docker's answer: storage that lives outside the container's own disposable filesystem, so it survives even after the container using it is removed.

docker run -v my-data:/var/lib/data my-app
# my-data is a named volume Docker manages for you
# /var/lib/data is the folder INSIDE the container that gets backed by it

There's also a simpler version, a bind mount, where instead of a Docker-managed volume you point directly at a real folder on your own machine - handy in local development when you want to edit a file on your laptop and see the change immediately inside the container.

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Environment Variables

Almost every real app needs some configuration - a database address, a feature flag, an API key - and you don't want to hard-code that inside the image itself, because then changing a setting means rebuilding the whole thing.

docker run -e LOG_LEVEL=debug -e DATABASE_URL=postgres://... my-app

Inside your Dockerfile, you can also set a default with ENV:

ENV LOG_LEVEL=info

A value passed with -e at docker run time overrides whatever ENV set in the Dockerfile - so the image has a sensible default, but anyone running it can still adjust it without rebuilding anything.

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Docker Compose Basics

Most real apps aren't just one container - a typical setup might need your app, a database, and a cache, all running together and able to talk to each other. Starting each one by hand with a long docker run command gets old fast, and it's easy to forget a flag.

Docker Compose lets you describe your whole setup in one file and start everything with one command.

services:
  web:
    build: .
    ports:
      - "8080:3000"
  database:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: example

docker compose up          # build/start everything described above
docker compose down        # stop and remove it all
docker compose logs -f      # follow logs from every service at once

Notice that web can reach database just by that name - Compose sets up networking between your services automatically, so you never need to hunt down an IP address.

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Cleaning Up

Docker doesn't delete things automatically, so unused images, stopped containers, and old volumes quietly pile up over weeks of normal use - it's one of the most common "why is my disk full?" surprises for anyone new to it.

docker container prune      # remove all stopped containers
docker image prune          # remove images no longer used by any container
docker volume prune          # remove volumes no longer used by any container
docker system prune -a       # the big one - clean up everything unused at once

docker system prune -a is genuinely useful to know early, if only so a slow laptop doesn't become a mystery.