Add Dockerfile for internal_frontend production deployment

internal_frontend/Dockerfile

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+# Use a lightweight Node.js 20 Alpine image (digest-pinned for reproducibility)
+FROM node:20-alpine@sha256:9bef0ef1e268f60627da9ba7d7605e8831d5b56ad07487d24d1aa386336d1944
+
+# Set the working directory inside the container
+WORKDIR /app
+
+# Copy only the production-ready files built in CI
+# These files must be passed as GitLab artifacts:
+# - .next/         → built Next.js app
+# - public/        → static assets
+# - package.json   → app manifest
+# - package-lock.json → to lock dependencies for reproducibility
+COPY .next .next
+COPY public public
+COPY package.json package.json
+COPY package-lock.json package-lock.json
+
+# Install only production dependencies to reduce image size
+RUN npm ci --omit=dev
+
+# Expose the port that Next.js serves on (default is 3000)
+EXPOSE 3000
+
+# Start the Next.js app in production mode
+CMD ["npm", "run", "start"]