diff --git a/production.Dockerfile b/production.Dockerfile
index 6ad538f..947b156 100644
--- a/production.Dockerfile
+++ b/production.Dockerfile
@@ -32,8 +32,9 @@ COPY . .
 ARG SECRET_KEY_BASE=fakekeyforassets
 RUN bin/rails assets:clobber && bundle exec rails assets:precompile
 
-# For when deploying to Render. It is not great, maybe there's a better way?
+# Run database migrations when deploying to Render. It is not great, maybe there's a better way?
 # https://community.render.com/t/release-command-for-db-migrations/247/6
+ARG RENDER
 ARG DATABASE_URL
 RUN printenv
 RUN if [ -z "$RENDER" ]; then echo "var is unset"; else bin/rails db:migrate; fi