Pin Gemfile.lock; restore proper Docker cache invalidation

Docker can't detect external git changes — Gemfile.lock change is
the signal. Workflow: after pushing rubbs, run bundle update bbs
and commit the new Gemfile.lock.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -4,7 +4,7 @@ RUN apk add --no-cache git build-base
 
 WORKDIR /app
 
-COPY Gemfile ./
+COPY Gemfile Gemfile.lock ./
 RUN bundle install
 
 COPY bbs.rb ./