build: fixes building grafana completely within docker.

Fixes #15060

Dockerfile

@@ -19,11 +19,13 @@ COPY package.json package.json
 RUN go run build.go build
 
 # Node build container
-FROM node:8
+FROM node:10.14.2
 
 WORKDIR /usr/src/app/
 
 COPY package.json yarn.lock ./
+COPY packages packages
+
 RUN yarn install --pure-lockfile --no-progress
 
 COPY Gruntfile.js tsconfig.json tslint.json ./