chore: harden build and deployment workflow

frontend/.dockerignore

@@ -0,0 +1,3 @@
+node_modules
+dist
+npm-debug.log*

frontend/Dockerfile

@@ -0,0 +1,13 @@
+FROM node:20-alpine AS build
+
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY --from=build /app/downloads /runtime/downloads
+
+EXPOSE 80