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 ./

Gemfile.lock

@@ -1,6 +1,6 @@
 GIT
   remote: https://git.teletype.hu/tools/rubbs.git
-  revision: 272e75bd6f90a235eeac684d412ceb04c146c926
+  revision: c01159dc2306ed9adda3b865346ae1f987649a67
   specs:
     bbs (0.1.0)
       artii (~> 2.1)