caddy-frontend: Use --upload-file for curl

It does -X PUT + --data-binary @file.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:30

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:18:14

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

08:37:19