assets:precompile on ci servers

example.rb

# in config/environments/test.rb
if running_on_ci?
  config.assets.compile = true
  config.assets.digest = true
end

# cache these two dirs:
# - tmp/cache/assets/test/sprockets
# - public/assets

# before test step compile assets to avoid on the fly compilation
# and very slow first acceptance test for each container
rake assets:precompile