"It works on my machine" — the most dreaded phrase in software development. Docker eliminates this problem entirely. When your app runs in a Docker container, it runs the same way everywhere: on your laptop, on your colleague's Mac, on a Linux server in AWS.
Key Concepts in Plain Language
Image — a blueprint (recipe) for your container. Think of it as a snapshot of a configured system.
Container — a running instance of an image. You can run multiple containers from the same image.
Dockerfile — a text file with instructions on how to build your image.
Docker Compose — a tool for running multiple containers together (e.g., your app + database + Redis).
A Real Dockerfile Example
Here's a Dockerfile for a Rails application:
FROM ruby:3.3
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install
COPY . .
EXPOSE 3000
CMD ["rails", "server", "-b", "0.0.0.0"]
Six lines that package your entire application into a portable container.
Docker Compose: The Real Power
Most applications need more than one service. Docker Compose lets you define everything in one file:
services:
web:
build: .
ports:
- "3000:3000"
depends_on:
- db
db:
image: postgres:16
volumes:
- pgdata:/var/lib/postgresql/data
Run docker compose up and your entire stack starts in seconds.
When to Use Docker
- Your team works on different operating systems
- You need consistent environments across dev, staging, and production
- You want to simplify deployment and scaling
- You're setting up a CI/CD pipeline
If you're still installing dependencies manually on each server, Docker will change your workflow forever.
Need help containerizing your application? Let's talk.
