Docker Deep Dive

An image is a read-only template - application code, a runtime, and every dependency, frozen in place. A container is a running instance of that image. The relationship is the same as a class and an object: one image, as many running containers from it as you like.

Layers

A Docker image is built from stacked, cacheable layers - each instruction in a Dockerfile produces one:

FROM node:20-alpine        # base layer
COPY package.json .        # a layer
RUN npm install            # a layer
COPY . .                   # a layer
CMD ["node", "server.js"]  # metadata, not a layer

If only your application code changes, Docker reuses the cached npm install layer from before instead of redoing it - which is exactly why COPY package.json and npm install are deliberately placed before copying the rest of the source: dependencies change far less often than application code, so that expensive layer gets reused far more often too.

Reading a Dockerfile

• FROM - the base image to build on top of.
• RUN - executes a command while building the image (installing packages, compiling).
• COPY - copies files from your machine into the image.
• CMD - the command that runs when a container starts from this image.

What docker run actually does

docker run -d --name web nginx

Pulls the nginx image if it isn't already local, creates a new container from it, names it web, and (-d) runs it detached - in the background, handing you your prompt straight back instead of attaching to its output.

Try it

docker run -d --name web nginx

When you're ready, hit Mark complete and move to the next lesson.