fix(docker): update frontend Dockerfile for Podman compatibility

- Remove deprecated --only=production flag
- Add default value for NEXT_PUBLIC_API_URL build arg
- Add BuildKit cache mount for npm
- Remove HEALTHCHECK (not supported by Podman OCI format)

frontend/Dockerfile

@@ -1,6 +1,7 @@
 # =============================================================================
 # GoHorse Jobs Frontend - Optimized Production Dockerfile
 # =============================================================================
+# syntax=docker/dockerfile:1
 
 # -----------------------------------------------------------------------------
 # Stage 1: Dependencies
@@ -9,10 +10,10 @@ FROM mirror.gcr.io/library/node:20-alpine AS deps
 
 WORKDIR /app
 
-# Install dependencies only when needed
+# Install ALL dependencies (dev + prod) for build stage
 COPY package.json package-lock.json* ./
-RUN npm ci --only=production --ignore-scripts && \
-    npm cache clean --force
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci && npm cache clean --force
 
 # -----------------------------------------------------------------------------
 # Stage 2: Builder
@@ -25,8 +26,8 @@ WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 
-# Build arguments for environment
-ARG NEXT_PUBLIC_API_URL
+# Build arguments for environment (with sensible default)
+ARG NEXT_PUBLIC_API_URL=http://localhost:8080
 ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
 
 # Build the application
@@ -61,8 +62,4 @@ USER nextjs
 
 EXPOSE 3000
 
-# Health check
-HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
-    CMD wget -qO- http://localhost:3000 || exit 1
-
 CMD ["node", "server.js"]