Docker Image Size Optimization

Analyzes Docker images and provides actionable recommendations to reduce footprint for PHP projects.

Image Size Anatomy

┌─────────────────────────────────────────────────────────────────┐
│              TYPICAL PHP IMAGE SIZE BREAKDOWN                    │
├─────────────────────────────────────────────────────────────────┤
│  Base OS (Debian):   ████████████████████░░░░░  150MB (33%)    │
│  PHP runtime:        ████████░░░░░░░░░░░░░░░░░   50MB (11%)   │
│  PHP extensions:     ██████░░░░░░░░░░░░░░░░░░░   40MB  (9%)   │
│  Build dependencies: ████████████░░░░░░░░░░░░░   80MB (18%)   │
│  Composer vendor:    ████████████░░░░░░░░░░░░░   80MB (18%)   │
│  Application source: ████░░░░░░░░░░░░░░░░░░░░░   20MB  (4%)   │
│  Misc (logs, cache): ████░░░░░░░░░░░░░░░░░░░░░   30MB  (7%)   │
│  Total unoptimized:  █████████████████████████  ~450MB          │
│  Target optimized:   ████████████░░░░░░░░░░░░░  ~120MB (-73%) │
└─────────────────────────────────────────────────────────────────┘

Size Estimation by Component

Component	Debian	Alpine	Savings
Base OS	150MB	8MB	-142MB
PHP runtime	50MB	35MB	-15MB
PHP extensions (5 typical)	40MB	25MB	-15MB
Build dependencies	80MB	0MB (multi-stage)	-80MB
Vendor (with dev)	80MB	30MB (--no-dev)	-50MB
Application source	20MB	10MB (.dockerignore)	-10MB
Total	~450MB	~108MB	-342MB

Alpine vs Debian

# ❌ Debian-based: ~150MB base → Final: 400-500MB
FROM php:8.4-fpm

# ✅ Alpine-based: ~8MB base → Final: 80-150MB
FROM php:8.4-fpm-alpine

Concern	Solution
musl vs glibc	Most PHP extensions work fine with musl
Missing packages	Use apk search to find Alpine equivalents
DNS resolution	Add gnu-libc-compat if needed

Multi-Stage Builds: Production-Only Artifacts

# Stage 1: Build with all tools
FROM php:8.4-cli-alpine AS builder
RUN apk add --no-cache --virtual .build-deps $PHPIZE_DEPS libzip-dev icu-dev \
    && docker-php-ext-install pdo_mysql intl zip
COPY --from=composer:2 /usr/bin/composer /usr/bin/composer
COPY composer.json composer.lock ./
RUN composer install --no-dev --prefer-dist --optimize-autoloader
COPY . .

# Stage 2: Minimal production image
FROM php:8.4-fpm-alpine AS production
RUN apk add --no-cache libzip icu-libs
COPY --from=builder /usr/local/lib/php/extensions/ /usr/local/lib/php/extensions/
COPY --from=builder /usr/local/etc/php/conf.d/ /usr/local/etc/php/conf.d/
COPY --from=builder /app/vendor /app/vendor
COPY --from=builder /app/src /app/src
COPY --from=builder /app/public /app/public
COPY --from=builder /app/config /app/config

Build Dependencies Cleanup

# ❌ BAD: Build deps remain in image (+80MB)
RUN apk add $PHPIZE_DEPS libzip-dev icu-dev \
    && docker-php-ext-install pdo_mysql intl zip

# ✅ GOOD: Virtual package for clean removal
RUN apk add --no-cache --virtual .build-deps \
        $PHPIZE_DEPS libzip-dev icu-dev libpng-dev \
    && docker-php-ext-install pdo_mysql intl zip gd \
    && apk del .build-deps \
    && apk add --no-cache libzip icu-libs libpng

Debian equivalent:

RUN apt-get update \
    && apt-get install -y --no-install-recommends libzip-dev libicu-dev \
    && docker-php-ext-install pdo_mysql intl zip \
    && apt-get purge -y --auto-remove libzip-dev libicu-dev \
    && rm -rf /var/lib/apt/lists/*

Composer Dependency Minimization

# ❌ BAD: Includes dev deps (+50MB: phpunit, phpstan, faker)
RUN composer install

# ✅ GOOD: Production only
RUN composer install --no-dev --prefer-dist --optimize-autoloader \
    --classmap-authoritative --no-scripts

Image Analysis Tools

# Dive: interactive layer analysis
dive app:latest

# CI mode: fail if wasted space > 10%
CI=true dive app:latest --highestWastedBytes 10MB

# Docker history: show layer sizes
docker history --human --no-trunc app:latest

Specific Savings Per Technique

Technique	Typical Savings	Effort
Switch to Alpine	120-200MB	Low
Multi-stage build	50-150MB	Medium
Remove build deps (.build-deps)	40-80MB	Low
Composer --no-dev	30-60MB	Low
.dockerignore	10-50MB	Low
Remove unused system packages	10-30MB	Low
Optimize COPY (specific paths)	5-20MB	Low
Strip debug symbols	5-15MB	Low

Before/After Comparison

Metric	Unoptimized	Optimized	Improvement
Base image	php:8.4-fpm (150MB)	php:8.4-fpm-alpine (8MB)	-142MB
Build deps	Included (80MB)	Multi-stage removed	-80MB
Vendor	With dev (80MB)	--no-dev (30MB)	-50MB
Source	Full repo (20MB)	.dockerignore (10MB)	-10MB
Total	~450MB	~108MB	-76%

Generation Instructions

1. Analyze current image: Run docker history and dive to identify large layers
2. Identify waste: Build deps, dev packages, unnecessary files
3. Apply techniques: Alpine base, multi-stage, cleanup, --no-dev
4. Measure result: Compare before/after with docker images
5. Validate runtime: Ensure application works correctly with minimal image