Docker Build Overview

Docker Build is the feature that allows you to create Docker images from a Dockerfile. Modern Docker builds are powered by BuildKit, which provides improved performance, caching, and features compared to the legacy builder.

How Docker Build Works

When you run docker build, the following process occurs:

┌───────────────────┐
│   Dockerfile      │
│   + Build Context │
└────────┬──────────┘
         │
         ▼
┌───────────────────┐     ┌──────────────────────────┐
│   Docker Client   │────▶│      BuildKit Backend    │
│   (docker build)  │     │                          │
└───────────────────┘     │  1. Parse Dockerfile     │
                          │  2. Create build graph   │
                          │  3. Execute stages        │
                          │  4. Cache intermediate    │
                          │  5. Export final image    │
                          │                          │
                          └────────────┬─────────────┘
                                       │
                                       ▼
                          ┌──────────────────────────┐
                          │     Docker Image          │
                          │  (stored in image store)  │
                          └──────────────────────────┘

Build Command

Basic Usage

# Build from current directory
docker build -t my-app:1.0 .

# Build with a specific Dockerfile
docker build -f Dockerfile.prod -t my-app:prod .

# Build with build arguments
docker build --build-arg NODE_ENV=production -t my-app:1.0 .

# Build without cache
docker build --no-cache -t my-app:1.0 .

# Build and show full output
docker build --progress=plain -t my-app:1.0 .

# Build for a specific platform
docker build --platform linux/amd64 -t my-app:1.0 .

# Build and push in one step (using buildx)
docker buildx build --push -t myuser/my-app:1.0 .

Build Command Options

Option	Description	Example
-t, --tag	Name and tag for the image	-t my-app:1.0
-f, --file	Path to Dockerfile	-f Dockerfile.prod
--build-arg	Set build-time variable	--build-arg VERSION=2.0
--no-cache	Do not use cache	--no-cache
--pull	Always pull base image	--pull
--target	Set target build stage	--target production
--platform	Set target platform	--platform linux/amd64
--progress	Set output type	--progress=plain
--secret	Expose a secret to the build	--secret id=key,src=key.pem
--ssh	SSH agent forwarding	--ssh default
--cache-from	External cache source	--cache-from type=registry,ref=cache:latest
--cache-to	External cache destination	--cache-to type=registry,ref=cache:latest
--output	Output destination	--output type=local,dest=./out
--load	Load into Docker images	--load
--push	Push to registry	--push

Build Context

The build context is the set of files sent to the Docker daemon for the build. By default, it is the current directory.

Context Types

# Local directory (most common)
docker build -t my-app:1.0 .

# Local directory with specific path
docker build -t my-app:1.0 /path/to/project

# Git repository
docker build -t my-app:1.0 https://github.com/user/repo.git

# Git with specific branch/tag
docker build -t my-app:1.0 https://github.com/user/repo.git#main

# Tarball URL
docker build -t my-app:1.0 https://example.com/context.tar.gz

# Stdin (no context)
docker build -t my-app:1.0 - < Dockerfile

# Stdin with context from tar
tar czf - . | docker build -t my-app:1.0 -

.dockerignore

The .dockerignore file excludes files from the build context:

# .dockerignore

# Version control
.git
.gitignore
.svn

# Dependencies
node_modules
vendor
__pycache__

# IDE and editor files
.vscode
.idea
*.swp
*.swo

# Build artifacts
dist
build
*.o
*.a

# Docker files
Dockerfile*
docker-compose*.yml
.dockerignore

# Documentation
*.md
docs/
LICENSE

# Test files
tests/
test/
coverage/
.nyc_output/

# Environment and secrets
.env
.env.*
*.pem
*.key
secrets/

# OS files
.DS_Store
Thumbs.db

Context Size Impact

Context Size	Build Time Impact	Recommendation
< 10 MB	Minimal	Good
10-100 MB	Noticeable	Optimize .dockerignore
100+ MB	Significant	Must optimize
1+ GB	Very slow	Critical issue

# Check your build context size
du -sh --exclude=.git .

# See what's being sent as context
docker build --no-cache --progress=plain -t test . 2>&1 | head -5

Docker Buildx

Docker Buildx is an extended build tool that provides additional features on top of BuildKit:

# Check buildx is available
docker buildx version

# List builders
docker buildx ls

# Create a new builder
docker buildx create --name mybuilder --use

# Inspect a builder
docker buildx inspect mybuilder

# Build for multiple platforms simultaneously
docker buildx build \
  --platform linux/amd64,linux/arm64,linux/arm/v7 \
  -t myuser/my-app:1.0 \
  --push .

Multi-platform Builds

# Create a multi-platform builder
docker buildx create --name multiplatform --driver docker-container --use

# Build for multiple architectures
docker buildx build \
  --platform linux/amd64,linux/arm64 \
  -t myuser/my-app:1.0 \
  --push .

# Inspect the multi-platform manifest
docker buildx imagetools inspect myuser/my-app:1.0

Platform Support Matrix

Platform	Architecture	Common Use Case
linux/amd64	x86_64	Standard servers, Intel/AMD desktops
linux/arm64	ARM 64-bit	AWS Graviton, Apple Silicon, Raspberry Pi 4
linux/arm/v7	ARM 32-bit	Raspberry Pi 3, older ARM devices
linux/arm/v6	ARM 32-bit	Raspberry Pi Zero/1
linux/386	x86 32-bit	Legacy systems
linux/s390x	IBM Z	IBM mainframes
linux/ppc64le	PowerPC	IBM Power systems

Build Output Types

# Load into local Docker images (default)
docker buildx build --load -t my-app:1.0 .

# Push directly to registry
docker buildx build --push -t myuser/my-app:1.0 .

# Export as a tar archive
docker buildx build --output type=tar,dest=image.tar .

# Export to local directory
docker buildx build --output type=local,dest=./output .

# Export as OCI image layout
docker buildx build --output type=oci,dest=image-oci.tar .

# Create image but don't export (cache only)
docker buildx build --output type=cacheonly .

Build Arguments and Variables

ARG vs ENV

Feature	ARG	ENV
Available during build	✅ Yes	✅ Yes
Available at runtime	❌ No	✅ Yes
Can be overridden at build	✅ Yes (--build-arg)	❌ No (set in Dockerfile)
Visible in image history	✅ Yes (⚠️ security risk)	✅ Yes
Scope	From definition to end of stage	From definition to container

# ARG: Build-time only
ARG NODE_VERSION=20
FROM node:${NODE_VERSION}-alpine

# ARG must be redeclared after FROM
ARG APP_VERSION=1.0

# ENV: Available at build and runtime
ENV NODE_ENV=production
ENV APP_VERSION=${APP_VERSION}

RUN echo "Building version ${APP_VERSION}"

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

# Override build arguments
docker build --build-arg NODE_VERSION=18 --build-arg APP_VERSION=2.0 -t my-app .

Build Secrets

# syntax=docker/dockerfile:1

FROM node:20-alpine

WORKDIR /app
COPY package.json package-lock.json ./

# Mount a secret file during build (not stored in image)
RUN --mount=type=secret,id=npmrc,target=/root/.npmrc \
    npm ci --only=production

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

# Pass the secret during build
docker build --secret id=npmrc,src=.npmrc -t my-app:1.0 .

SSH Forwarding

# syntax=docker/dockerfile:1

FROM alpine:3.19

RUN apk add --no-cache git openssh-client

# Use SSH agent forwarding to clone private repos
RUN --mount=type=ssh \
    git clone [email protected]:private/repo.git /app

# Build with SSH forwarding
docker build --ssh default -t my-app:1.0 .

Next Steps

• BuildKit Guide — Deep dive into BuildKit features
• Build Optimization — Optimize build performance
• Build Cache — Master build caching strategies
• Multi-stage Builds — Reduce image sizes
• Image Building Best Practices — Production-ready images