build(docker): switch to multistage build image

build/Containerfile

@@ -1,5 +1,20 @@
+ARG SITE=signpost.cloud.mareshq.com
+
+FROM node:lts as builder
+
+WORKDIR /build
+
+COPY ./sites/${SITE}/package.json .
+
+RUN yarn install --frozen-lockfile
+
+COPY ./sites/${SITE} .
+
+RUN yarn build
+
+# Final image
 FROM nginx:mainline-alpine as nginx
 
-COPY ./../../build/nginx/site.conf /etc/nginx/conf.d/default.conf
+COPY ./build/nginx/site.conf /etc/nginx/conf.d/default.conf
 
-COPY ./out /var/www/html
+COPY --from=builder /build/out /var/www/html