fix: add curl and pass environment variables to Docker build

- Install curl in Alpine image for health checks
- Add ARG and ENV declarations for NEXT_PUBLIC_* variables in builder stage
- Pass build args from docker-compose.yml to Dockerfile
- Update health checks to use curl instead of node -e
- Update .env.example with helpful comments for Docker vs local dev

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

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

Dockerfile

@@ -29,6 +29,14 @@ COPY . .
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
 
+# Accept build args for environment variables
+ARG NEXT_PUBLIC_API_URL
+ARG NEXT_PUBLIC_APP_URL
+
+# Set environment variables for Next.js build
+ENV NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL}
+ENV NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL}
+
 # Build the application
 RUN pnpm build
 
@@ -40,6 +48,9 @@ WORKDIR /app
 ENV NODE_ENV=production
 ENV NEXT_TELEMETRY_DISABLED=1
 
+# Install curl for health checks
+RUN apk add --no-cache curl
+
 # Create non-root user
 RUN addgroup --system --gid 1001 nodejs && \
     adduser --system --uid 1001 nextjs
@@ -58,6 +69,6 @@ ENV HOSTNAME="0.0.0.0"
 
 # Health check
 HEALTHCHECK --interval=30s --timeout=10s --start-period=40s --retries=3 \
-  CMD node -e "require('http').get('http://localhost:3000/api/health', (r) => {process.exit(r.statusCode === 200 ? 0 : 1)})"
+  CMD curl -f http://localhost:3000/ || exit 1
 
 CMD ["node", "server.js"]