Optimize Dockerfile with multi-stage build

- Added multi-stage build to reduce final image size
- Stage 1 (builder): Installs all deps, compiles TS, generates OpenAPI, builds HTML
- Stage 2 (production): Fresh base image with only production deps and compiled artifacts
- Final image no longer contains src/, tsconfig.json, or dev dependencies
- Added TDD test (dockerfile-build.test.ts) to verify build artifacts exist
- All 561 tests pass

Reduces image size by excluding TypeScript source, build tools, and dev dependencies.

Dockerfile

@@ -1,4 +1,37 @@
-FROM node:22-bookworm-slim
+# ============================================
+# Stage 1: Builder
+# ============================================
+FROM node:22-bookworm-slim AS builder
+
+WORKDIR /app
+
+# Copy package files for dependency installation
+COPY package*.json tsconfig.json ./
+
+# Install ALL dependencies (including devDependencies for build)
+RUN npm install
+
+# Copy source code and build scripts
+COPY src/ src/
+COPY scripts/ scripts/
+COPY public/ public/
+
+# Compile TypeScript
+RUN npx tsc
+
+# Generate OpenAPI spec
+RUN node scripts/generate-openapi.mjs
+
+# Build HTML templates
+RUN node scripts/build-html.cjs
+
+# Create swagger-ui symlink in builder stage
+RUN rm -f public/swagger-ui && ln -s /app/node_modules/swagger-ui-dist public/swagger-ui
+
+# ============================================
+# Stage 2: Production
+# ============================================
+FROM node:22-bookworm-slim AS production
 
 # Install Chromium and dependencies as root
 RUN apt-get update && apt-get install -y --no-install-recommends \
@@ -9,25 +42,25 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN groupadd --gid 1001 docfast \
   && useradd --uid 1001 --gid docfast --shell /bin/bash --create-home docfast
 
-# Set environment variables
+WORKDIR /app
+
+# Copy package files for production dependency installation
+COPY package*.json ./
+
+# Install ONLY production dependencies
+RUN npm install --omit=dev
+
+# Copy compiled artifacts from builder stage
+COPY --from=builder /app/dist ./dist
+COPY --from=builder /app/public ./public
+
+# Recreate swagger-ui symlink in production stage
+RUN rm -f public/swagger-ui && ln -s /app/node_modules/swagger-ui-dist public/swagger-ui
+
+# Set Puppeteer environment variables
 ENV PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true
 ENV PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium
 
-# Build stage - compile TypeScript
-WORKDIR /app
-COPY package*.json tsconfig.json ./
-RUN npm install
-COPY src/ src/
-RUN npx tsc
-
-# Remove dev dependencies
-RUN npm prune --omit=dev
-COPY scripts/ scripts/
-COPY public/ public/
-RUN node scripts/generate-openapi.mjs
-RUN node scripts/build-html.cjs
-RUN rm -f public/swagger-ui && ln -s /app/node_modules/swagger-ui-dist public/swagger-ui
-
 # Create data directory and set ownership to docfast user
 RUN mkdir -p /app/data && chown -R docfast:docfast /app