Add a two-stages Dockerfile for building Riot

Dockerfile

@@ -0,0 +1,20 @@
+# Builder
+FROM node:alpine as builder
+
+COPY . /src
+
+WORKDIR /src
+
+RUN apk add --no-cache git \
+ && npm install \
+ && npm run build
+
+
+# App
+FROM nginx:latest
+
+COPY --from=builder /src/webapp /app
+COPY config.sample.json /app/config.json
+
+RUN rm -rf /usr/share/nginx/html \
+ && ln -s /app /usr/share/nginx/html