beamer-testbed runs with local user's UID/GID

so that created files aren't owned by root

Dockerfile

@@ -61,11 +61,7 @@ RUN apt-get update -qq && \
 COPY --from=build /usr/local/bundle /usr/local/bundle
 COPY --from=build /rails /rails
 COPY --from=beamer /home/beamer/bin/beamer.so /rails/bin/lib/beamer.so
-
-# Run and own only the runtime files as a non-root user for security
-RUN useradd rails --create-home --shell /bin/bash && \
-    chown -R rails:rails db log storage tmp
-USER rails:rails
+COPY --from=beamer /usr/local/bin/beamer /rails/bin/beamer
 
 # Entrypoint prepares the database.
 ENTRYPOINT ["/rails/bin/docker-entrypoint"]