fix: Docker build — fix /bundle permissions for bundle install

Replace cache mount (which doesn't fix ownership of existing subdirs)
with explicit chown before bundle install. Fixes permission error
when /bundle/ruby/4.0.0 was previously created by root.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

Dockerfile

@@ -51,7 +51,7 @@ WORKDIR /app
 # Copy entire project (needed for git dependencies in gems.rb)
 COPY --chown=circleci:circleci . .
 
-# Install gems with cached bundle
-RUN --mount=type=cache,target=/bundle,uid=1000,gid=1000 \
+# Install gems
+RUN sudo chown -R circleci:circleci /bundle && \
     bundle config set without 'tools' && \
     bundle install