A bare rackup is development, with Rack::Lint and ShowExceptions in the chain;
RACK_ENV set in the image leaves config.ru the only middleware to explain.

Assisted-by: Claude Opus 5 via Claude Code
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

diff --git a/Dockerfile b/Dockerfile
index 470a270..26b2808 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -21,6 +21,9 @@ ENV COMMIT_SHA=${COMMIT_SHA}
 ENV CHANGE_ID=${CHANGE_ID}
 ENV BUILD_DATE=${BUILD_DATE}
 ENV VERSION=${VERSION}
+# rackup defaults to development, which wraps the app in Rack::Lint and
+# ShowExceptions.
+ENV RACK_ENV=production
 # Set TZ at runtime — Date.today drives the birthday list's rollover.
 
 WORKDIR /app