Use existing bundle path that already declared (rails#50615)

Author: anonychun

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -87,7 +87,7 @@ RUN apt-get update -qq && \
 
 <% end -%>
 # Copy built artifacts: gems, application
-COPY --from=build /usr/local/bundle /usr/local/bundle
+COPY --from=build "${BUNDLE_PATH}" "${BUNDLE_PATH}"
 COPY --from=build /rails /rails
 
 # Run and own only the runtime files as a non-root user for security

railties/test/fixtures/Dockerfile.test

@@ -50,7 +50,7 @@ RUN apt-get update -qq && \
     rm -rf /var/lib/apt/lists /var/cache/apt/archives
 
 # Copy built artifacts: gems, application
-COPY --from=build /usr/local/bundle /usr/local/bundle
+COPY --from=build "${BUNDLE_PATH}" "${BUNDLE_PATH}"
 COPY --from=build /rails /rails
 
 # Run and own only the runtime files as a non-root user for security