DOCKER
YARD.

A working reference for how images get built, how containers run, and every command you'll reach for in between. Laid out like cargo on a dock — stacked, labeled, ready to move.

DOCKERFILE → IMAGE → CONTAINER 40+ COMMANDS

// image layers, bottom to top

CMD ["node","server.js"]L5

COPY . .L4

RUN npm installL3

COPY package.json .L2

FROM node:20-alpineL1

Structure

How an image is built

Every Dockerfile instruction adds one read-only layer on top of the last. Docker caches each layer, so ordering them from least-changing (dependencies) to most-changing (your source code) keeps rebuilds fast.

FROMSets the base image everything else builds on.

WORKDIRSets the working directory for the instructions that follow.

COPY / ADDCopies files from the build context into the image. ADD also handles remote URLs and archive extraction.

RUNExecutes a command at build time and commits the result as a new layer.

ENV / ARGENV sets runtime environment variables. ARG sets build-time-only variables.

EXPOSEDocuments which port the container listens on. Doesn't publish it.

VOLUMEMarks a path as a mount point for persistent or shared data.

USERSets the user the container runs as, instead of root.

CMDDefault command for the container. Overridden by args passed to `docker run`.

ENTRYPOINTFixed executable the container always runs. CMD supplies default arguments to it.

HEALTHCHECKDefines a command Docker runs periodically to judge if the container is healthy.

// container = image + writable layer

Writable container layerRW

CMD ["node","server.js"]L5

COPY . .L4

RUN npm installL3

FROM node:20-alpineL1–2

Flow

Where each command sits

The same image moves through three states: built once, run many times, and shared through a registry. Compose sits alongside this for multi-container setups.

Dockerfile

Source of truth for the image.

build

Image

Immutable, versioned, taggable.

docker build -t app:1.0 .

run

Container

A running instance of the image.

docker run -d -p 3000:3000 app:1.0

push

Registry

Where images are stored and pulled from.

docker push app:1.0

Compose File

Running more than one container

docker-compose.yml describes a whole stack — every service, how they connect, and what they depend on — so `docker compose up` can start it all at once.

docker-compose.yml

services:
  web:
    build: .
    ports:
      - "3000:3000"
    environment:
      - NODE_ENV=production
    depends_on:
      - db
    volumes:
      - ./src:/app/src

  db:
    image: postgres:16
    environment:
      - POSTGRES_PASSWORD=secret
    volumes:
      - db-data:/var/lib/postgresql/data

volumes:
  db-data:

servicesTop-level list of containers that make up the app. Each key becomes a container name.

buildBuilds an image from a local Dockerfile instead of pulling one.

imageUses an existing image from a registry instead of building.

portsMaps host_port:container_port so a service is reachable from outside.

environmentSets environment variables inside the container.

volumesMounts a host path or named volume into the container for persistent data.

depends_onControls startup order. Doesn't wait for the other service to be ready, only started.

networksAttaches the service to a custom network so services can reach each other by name.

DOCKERYARD.