Stop 04 · Compute ServicesTopics · 5/10

Containers Overview (Docker Basics)

In short

'It works on my machine' has ruined many demos. Containers pack your app with everything it needs, so it runs the same on your laptop, your friend's laptop and a cloud server.

Packaging an application and its dependencies into a portable, isolated unit — a lighter-weight alternative to a full VM, and the foundation for the container orchestration covered in Phase 9.

The blob character packs food into a tall stacked steel tiffin.

Think of it like this

The steel tiffin

Amma packs rice, dal and sabzi into one sealed tiffin. Whether it's opened in the office, on the train or in the hostel, it's the same lunch. A container is a tiffin for your app and its ingredients.

Watch it happen

From code to running container

Dockerfile

Build

Image

Registry

Run

1/5A Dockerfile lists the steps: base image, copy code, install packages, start command.

The big ideas

What you need to know

Image vs container

The image is the recipe and packed ingredients. A container is one running copy of it. One image, many containers.

Lighter than a VM

Containers share the host's kernel instead of booting their own OS, so they start fast and pack densely.

Isolated, not separate machines

Each container has its own files, processes and network, but they share the host kernel.

Same everywhere

The same image runs on your laptop, in CI and in the cloud. Fewer 'but it worked yesterday' surprises.

See it in code

Walk through the code

dockerfile

1FROM node:22-alpine2WORKDIR /app3COPY package*.json ./4RUN npm ci --omit=dev5COPY . .6EXPOSE 30007CMD ["node", "server.js"]8 9# docker build -t notes-api:1.0 .10# docker run -p 3000:3000 notes-api:1.0

1/4Start from a small official Node image and work inside /app.

Copying package.json before the rest means Docker reuses the cached npm install layer when only your code changes.

Save yourself the pain

Common mistakes

Oops

Using the :latest tag in production.

Do this instead

Tag images with a version or git commit. :latest changes under you and makes rollbacks guesswork.

Oops

Baking passwords and API keys into the image.

Do this instead

Pass secrets at runtime through environment variables or a secrets manager. Anyone with the image can read its layers.

Your mission

Containerise a tiny app

~30 min

Quick check

Why do containers usually start faster than VMs?

Go deeper

1Install Docker and run your first container from a public image.
2Learn the difference between a VM (full OS) and a container (shares the host OS kernel).
3Build a simple Dockerfile for a small application.