feat: add Dockerfile with Node 18 Alpine, non-root user, health check

Dockerfile

@@ -0,0 +1,30 @@
+FROM node:18-alpine
+
+LABEL maintainer="ImmichFrame"
+LABEL description="Lightweight digital photo frame for Immich"
+
+WORKDIR /app
+
+# Copy package files first for better caching
+COPY package.json ./
+
+# Install dependencies
+RUN npm install --production && npm cache clean --force
+
+# Copy application
+COPY server.js ./
+COPY public/ ./public/
+
+# Expose port
+EXPOSE 3000
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD wget -qO- http://localhost:3000/api/config || exit 1
+
+# Run as non-root
+RUN addgroup -g 1001 -S appgroup && \
+    adduser -S appuser -u 1001 -G appgroup
+USER appuser
+
+CMD ["node", "server.js"]