caddy-frontend: Cover failover URL with cache

Configuring failover URL and being able to rely on cached content is not so
simple, thus document the requirement and prove in test, that such
configuration works correctly.
5 jobs for feat/theia-1.31.1
Status Job ID Name Coverage
  External
passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

failed SlapOS.SoftwareReleases.IntegrationTest-Master

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch