Add --frozen-lockfile to docker image yarn install

Webpacker 5.1.0 removed yarn integrity checker. This commit ensures that
we also use the yarn.lock we generated while developing rather than updating
it. It should also fail if package.json & yarn.lock are out of sync.

Yarn's own maintainer discourages its use:
yarnpkg/yarn#6427 (comment)
and suggests the following solution:
Solution
Remove the integrity initializer and the configuration options. Encourage
developers to ensure yarn install in local environments and
yarn install --frozen-lockfile as part of deployment.

Other references:
https://classic.yarnpkg.com/en/docs/cli/install#toc-yarn-install-frozen-lockfile
rails/webpacker#2518

Dockerfile

@@ -21,7 +21,7 @@ RUN apk add --update --no-cache --virtual build-dependances \
 
 ADD package.json $APP_HOME/package.json
 ADD yarn.lock $APP_HOME/yarn.lock
-RUN yarn
+RUN yarn install --frozen-lockfile
 
 ADD . $APP_HOME/