Fix frontend container build

packages/lesspass-site/Dockerfile

@@ -1,9 +1,10 @@
-FROM node:lts AS builder
+FROM node:12-alpine AS builder
 LABEL maintainer="LessPass <contact@lesspass.com>"
 LABEL name="LessPass Frontend"
 WORKDIR /opt/frontend
 COPY package.json yarn.lock ./
 RUN yarn install
+RUN yarn global add gulp-cli
 COPY . /opt/frontend
 RUN yarn build
 FROM nginx:alpine