Write a Dockerfile Without Reading the Reference

Choose the runtime, say how the app installs and starts, and the Dockerfile on the right updates as you go. The defaults are the ones that keep images small and builds cacheable.

Runtime

Base image and files

Base image (name:tag) Working directory

Dependency manifest(s) copied first, so installs are cached until they change (comma separated) Install command Build command (optional, runs after the source is copied) System packages to apt-get or apk (optional, space separated)

How it runs

Start command Port the app listens on

Environment defaults, KEY=value, one per line (optional) Run as a non-root user Creates an app user, hands it the working directory and switches to it before CMD. A container escape from root is a host compromise; from an unprivileged user it is a nuisance. Add a HEALTHCHECK An HTTP probe against the port above every 30 seconds. Compose and orchestrators use it to know when the container is actually ready, not just started. Multi-stage build Installs and compiles in a build stage, then copies only the result into a clean runtime image. Dev dependencies, compilers and caches never reach production. Comment each step One line above every instruction saying why it is there and in that order.

Dockerfile

.dockerignore keeps the build context small

Why the Dockerfile Is Ordered This Way

Docker caches each instruction as a layer and reuses it until something that instruction depends on changes. Copying package.json and running the install before copying the rest of the source means a one-line code change rebuilds in seconds, because the install layer is still valid. Copy everything first and every build reinstalls every dependency. That single ordering choice is the difference between a 5-second and a 5-minute build, and the generator does it for every runtime.

The multi-stage option does the install and any compile step in a builder stage, then starts a fresh runtime image and copies only the output across. Compilers, dev dependencies and package caches stay behind. For Go and static sites the final image can be a few megabytes; for Node and Python it removes the build toolchain that a single-stage image drags along. Package installs get a cleanup on the same RUN line (apt lists removed, apk --no-cache), because a delete in a later layer does not shrink the earlier one.

Non-root and healthchecks

Containers run as root by default, and most applications have no reason to. The generated file creates a user, gives it the working directory, and switches with USER just before CMD, so the install steps that need root still work. The HEALTHCHECK line polls the app's port; docker ps then shows healthy instead of merely up, and a Compose file can make other services wait for it. The guide explains how the two files work together, and the validator reviews any existing Dockerfile line by line.