use env.DB_NAME to access the environment variable

Jenkinsfile

@@ -10,7 +10,7 @@ pipeline {
         sh ''' #!/usr/local/bin/bash
           rbenv local 3.2.2
           echo "ENV[\\\"APP_SESSION_SECRET\\\"] ||= $(ruby -rsecurerandom -e 'puts SecureRandom.base64(64).inspect()')" > .env.rb
-          echo 'ENV["DB_NAME"] ||= "${DB_NAME}"' >> .env.rb
+          echo 'ENV["DB_NAME"] ||= "${env.DB_NAME}"' >> .env.rb
         '''
       }
     }