fix(docker): add canvas native deps for jsdom/vitest

canvas is an optional dependency of jsdom (used by vitest) and requires
native libraries (cairo, pango, pixman) to build on Alpine. Add these
dependencies to the base stage.

Also fix legacy ENV format warnings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -1,8 +1,20 @@
 # Multi-stage build for Soroban Abacus Flashcards
 FROM node:20-alpine AS base
 
-# Install Python and build tools for better-sqlite3
-RUN apk add --no-cache python3 py3-setuptools make g++
+# Install Python, build tools for better-sqlite3, and canvas native dependencies
+# canvas is an optional dep of jsdom (used by vitest) and requires cairo/pango
+RUN apk add --no-cache \
+    python3 \
+    py3-setuptools \
+    make \
+    g++ \
+    pkgconfig \
+    cairo-dev \
+    pango-dev \
+    libjpeg-turbo-dev \
+    giflib-dev \
+    librsvg-dev \
+    pixman-dev
 
 # Install pnpm and turbo
 RUN npm install -g pnpm@9.15.4 turbo@1.10.0
@@ -155,9 +167,9 @@ RUN mkdir -p data/uploads && chown -R nextjs:nodejs data
 
 USER nextjs
 EXPOSE 3000
-ENV PORT 3000
-ENV HOSTNAME "0.0.0.0"
-ENV NODE_ENV production
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
+ENV NODE_ENV=production
 
 # Start the application
 CMD ["node", "server.js"]