Fix docker issue adding --frozen

--frozen prevents Gemfile.lock from being updated,
which was causing write errors for me.

Also renames the image to lsm-hpce/docs

Dockerfile

@@ -3,7 +3,7 @@ FROM jekyll/builder
 WORKDIR /tmp
 ADD Gemfile /tmp/
 ADD Gemfile.lock /tmp/
-RUN bundle install
+RUN bundle install --frozen
 
 FROM jekyll/jekyll