Update docker.md (#2707)

"ENV key=value" should be used instead of legacy "ENV key value" format

Author: the-clint

docusaurus/docs/cms/installation/docker.md

@@ -73,7 +73,7 @@ WORKDIR /opt/
 COPY package.json yarn.lock ./
 RUN yarn global add node-gyp
 RUN yarn config set network-timeout 600000 -g && yarn install
-ENV PATH /opt/node_modules/.bin:$PATH
+ENV PATH=/opt/node_modules/.bin:$PATH
 
 WORKDIR /opt/app
 COPY . .