Add --force option to npm install commands in dockerfiles (microsoft#36431)

Author: weswigham

tests/cases/docker/office-ui-fabric/Dockerfile

@@ -1,5 +1,5 @@
 FROM node:current
-RUN npm install -g yarn lerna
+RUN npm install -g yarn lerna --force
 RUN git clone https://github.com/OfficeDev/office-ui-fabric-react.git /office-ui-fabric-react
 WORKDIR /office-ui-fabric-react
 RUN git pull

tests/cases/docker/vscode/Dockerfile

@@ -2,7 +2,7 @@
 FROM node:10
 RUN apt-get update
 RUN apt-get install libsecret-1-dev libx11-dev libxkbfile-dev -y
-RUN npm i -g yarn
+RUN npm i -g yarn --force
 RUN git clone https://github.com/microsoft/vscode.git /vscode
 WORKDIR /vscode
 RUN git pull