What Is Docker? The Fundamentals

Every developer has heard "works on my machine but not on the server." Docker was born to kill exactly that problem: it packages your app together with everything it needs to run (libraries, runtime, system tools) into one artifact that runs identically on your laptop, a server, or the cloud.

Image vs. container

Two core concepts. An image is a frozen template of your app — an immutable blueprint. A container is a live, running instance of that image. You can run as many containers as you want from one image — like creating many objects from one class.

Dockerfile: you describe the image

You describe how to build an image, step by step, in a text file called Dockerfile. For a sample Node.js app:

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

The commands you'll use most

• docker build -t myapp . → build an image from the Dockerfile
• docker run -p 3000:3000 myapp → start the container and publish the port
• docker ps → list running containers
• docker logs <id> → view a container's logs
• docker exec -it <id> sh → shell into a running container

Why it matters so much

Docker is the foundational building block of modern DevOps: Kubernetes orchestrates containers, CI/CD pipelines build images, cloud services run containers. Without understanding Docker you can't fully grasp Kubernetes or CI/CD.

Case: shrink a 900MB image to 90MB

The simple Dockerfile above works but is bloated: build tools, dev dependencies and source code all leak into the image. A prod image should be lean. Multi-stage builds solve exactly this.

$ docker build -t web:naive .
$ docker images web:naive
REPOSITORY   TAG     SIZE
web          naive   912MB          # build tools + node_modules + source, all inside
$ docker history web:naive | head -3   # where did the layers go?
<missing>  380MB  RUN npm install
<missing>  280MB  COPY . .

Multi-stage Dockerfile

# stage 1: build
FROM node:20 AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# stage 2: runtime only
FROM node:20-alpine
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
CMD ["node", "dist/server.js"]

$ docker build -t web:slim .
$ docker images web:slim
REPOSITORY   TAG    SIZE
web          slim   92MB           # ✓ 912MB → 92MB (~90% smaller)

The fastest way to learn is trying the commands in a terminal — which is exactly what Cloudpuz's interactive hands-on labs are built for.