diff --git a/Containerfile b/Containerfile
index 4d3a6049..637aebee 100644
--- a/Containerfile
+++ b/Containerfile
@@ -65,14 +65,13 @@ RUN bundle config set without 'development test'     \
  && yarn install --frozen-lockfile
 
 # temporarily set a SECRET_KEY_BASE and copy config files so rake tasks can run
-ENV SECRET_KEY_BASE=secret_for_build
+ARG SECRET_KEY_BASE=secret_for_build
 RUN cp config/justask.yml.example config/justask.yml    \
  && cp config/database.yml.postgres config/database.yml \
  && bundle exec rails locale:generate                   \
  && bundle exec i18n export                             \
  && bundle exec rails assets:precompile                 \
  && rm config/justask.yml config/database.yml
-ENV SECRET_KEY_BASE=
 
 # set some defaults
 ENV RAILS_LOG_TO_STDOUT=true