Building Docker Images

Running pre-built images is useful, but the real power of Docker comes from building your own images. This lesson covers Dockerfile syntax, key instructions, build context, layer caching, .dockerignore, and multi-stage builds.

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What Is a Dockerfile?

A Dockerfile is a plain text file containing a series of instructions that Docker uses to assemble an image. Each instruction creates a layer in the image.

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

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Key Dockerfile Instructions

FROM — Base Image

Every Dockerfile must start with FROM. This sets the base image that your image builds upon.

# Official Node.js image based on Alpine Linux (small)
FROM node:20-alpine

# Official Python image
FROM python:3.12-slim

# Minimal base — just a shell
FROM alpine:3.19

# Empty base — for fully static binaries
FROM scratch

Tip: Always use specific tags (e.g. node:20-alpine) rather than latest to ensure reproducible builds.

RUN — Execute Commands

RUN executes a command during the build and commits the result as a new layer.