Add missing Gemfile.lock to Docker install steps

Addressed using guidance from ***REMOVED***

b/270567886

Change-Id: I53fca0e9009ed17ad233fb2522cde9ad24e34872
NOKEYCHECK=True
GitOrigin-RevId: f4ba2e56e9f663e10eeac8f2e361588dd5b06778

docker/docsite/Dockerfile

@@ -34,7 +34,10 @@ RUN apt update -qqy \
     && rm -rf /var/lib/{apt,dpkg,cache,log}
 
 COPY Gemfile /app/Gemfile
-
+# Note: This file was generated by running a working version of this Docker
+# container. Then it was committed back to the codebase to be copied over as
+# part of the build steps.
+COPY Gemfile.lock /app/Gemfile.lock
 RUN bundle install --gemfile=/app/Gemfile
 
 # We create and use a non-root user explicitly so that the generated and