Don't use the --full-index option in Dockerfile

The --full-index option seemed to be causing caching issues on some
systems.

Since we don't know the reason why this option was added in the first
place, it might have some advantages. However, some people have reported
problems getting "version can no longer be found" errors for some gems
in this step, and documentation for Docker and Rails doesn't mention
this option at all.

Dockerfile

@@ -30,9 +30,7 @@ WORKDIR $RAILS_ROOT
 # Use the Gemfiles as Docker cache markers. Always bundle before copying app src.
 # (the src likely changed and we don't want to invalidate Docker's cache too early)
 COPY Gemfile* ./
-
-# Finish establishing our Ruby environment
-RUN bundle install --full-index
+RUN bundle install
 
 # Copy the Rails application into place
 COPY . .