fix: Docker build — restore .git and fix bundle cache permissions

- Remove .git/ from .dockerignore (needed for gemspec git ls-files
  and VCS checkout in tests)
- Add uid/gid to bundle cache mount so circleci user can write

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

Author: pftg

.dockerignore

@@ -2,7 +2,7 @@
 .ruby-lsp/
 coverage/
 tmp/
-.git/
+# Keep .git/ — needed for gemspec git ls-files and VCS checkout in tests
 .idea/
 .claude/
 .bundle/

Dockerfile

@@ -52,6 +52,6 @@ WORKDIR /app
 COPY --chown=circleci:circleci . .
 
 # Install gems with cached bundle
-RUN --mount=type=cache,target=/bundle \
+RUN --mount=type=cache,target=/bundle,uid=1000,gid=1000 \
     bundle config set without 'tools' && \
     bundle install