Add explicit error message for pages users using -domain-config-source
Summary
We are deprecating -domain-config-source in %14.0 . Users who will not change their config will see pages failing without understanding why. This issue address this problem by adding an explicit error message to the logs so they will know what to do to remedy this.
Proposal
Pages fails when a flag is defined and it doesn't exist with
$ ./gitlab-pages -config gitlab-pages.conf -invali-flag
flag provided but not defined: -invalid-flag
Usage of ./gitlab-pages:
# truncated ... it shows the available flags
We want to explicitly look for -domain-config-source
and have to handle the case explicitly and we can prevent Pages from starting with a message, for example:
$ ./gitlab-pages -config gitlab-pages.conf -domain-config-source=disk
FATA[0000] Please use API-based configuration https://docs.gitlab.com/ee/administration/pages/#gitlab-api-based-configuration error="DEPRECATED: -domain-config-source"
References
Coming from gitlab-com/www-gitlab-com!75474 (comment 524820846)