Fix frontend build

packages/lesspass-site/Dockerfile

@@ -2,7 +2,7 @@ FROM node:12-alpine AS builder
 LABEL maintainer="LessPass <contact@lesspass.com>"
 LABEL name="LessPass Frontend"
 WORKDIR /opt/frontend
-COPY package.json yarn.lock ./
+COPY package.json ./
 RUN yarn install
 RUN yarn global add gulp-cli
 COPY . /opt/frontend