perf: optimize Docker build with layer caching and BuildKit support

docker/Dockerfile

@@ -4,17 +4,21 @@
 FROM node:20-bullseye AS builder
 WORKDIR /app
 
-# Install dependencies
+# Copy ONLY package files first (creates cacheable layer)
 COPY package*.json ./
-RUN npm ci
 
-# Copy sources
+# Install dependencies with cache mount for faster rebuilds
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci
+
+# Copy source files AFTER npm install (only invalidates cache if code changes)
 COPY . .
 
-# Build Angular app (outputs to ./dist)
-RUN npx ng build --configuration=production
+# Build Angular app with cache mount for Angular's build cache
+RUN --mount=type=cache,target=/app/.angular/cache \
+    npx ng build --configuration=production
 
-# Prune dev dependencies to keep only production deps for runtime copy
+# Prune dev dependencies to keep only production deps
 RUN npm prune --omit=dev
 
 # -------------------- Runtime stage --------------------