yarn -> npm in dockerfile

support/docker/production/Dockerfile.stretch

@@ -1,6 +1,6 @@
 FROM node:8-stretch
 
-# Allow to pass extra options to the yarn run build
+# Allow to pass extra options to the npm run build
 # eg: --light --light-fr to not build all client languages
 #     (speed up build time if i18n is not required)
 ARG NPM_RUN_BUILD_OPTS
@@ -59,5 +59,5 @@ COPY ./support/docker/production/docker-entrypoint.sh /usr/local/bin/docker-entr
 ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
 
 # Run the application
-CMD ["yarn", "start"]
+CMD ["npm", "start"]
 EXPOSE 9000