Why Docker?

"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.

A Docker container packages your application with everything it needs: the operating system, runtime, libraries, and configuration. It's like shipping your entire development environment along with your code.

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. It's your recipe.

Docker Compose — a tool for running multiple containers together (e.g., your app + database + Redis).

A Real Dockerfile Example

Here's a simplified 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"]

That's it. 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 a single docker-compose.yml 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

Docker is ideal when:

  • 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.