perf: add Docker build cache for Next.js

Mount .next/cache as a BuildKit cache volume during build to persist
the Next.js build cache across Docker rebuilds. Eliminates the
'No build cache found' warning and speeds up subsequent builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Dockerfile

@@ -31,10 +31,10 @@ RUN npx prisma generate
 # Copy source code (this invalidates cache when code changes)
 COPY . .
 
-# Build the application
+# Build the application (mount cache for faster rebuilds)
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
-RUN npm run build
+RUN --mount=type=cache,target=/app/.next/cache npm run build
 
 # Verify standalone output was created and show structure for debugging
 RUN if [ ! -d .next/standalone ]; then \