Resolved. GitLab Pages doesn't work since GitLab v14.0

ref. #2375

As I can see from the error message below, it looks like both
`api-secret-key` and `gitlab-server` (or `internal-gitlab-server` ) are required since GitLab v14.0

```
time="2021-06-28T02:28:34Z" level=fatal msg="could not create domains config source" error="GitLab API URL or API secret has not been provided"
```

https://gitlab.com/gitlab-org/gitlab-pages/-/blob/v1.41.0/internal/source/gitlab/client/client.go#L50-52

Author: sue445

assets/runtime/config/gitlab-pages/config

@@ -4,3 +4,5 @@ auth-redirect-uri={{GITLAB_PAGES_ACCESS_REDIRECT_URI}}
 auth-secret={{GITLAB_PAGES_ACCESS_SECRET}}
 gitlab-server={{GITLAB_PAGES_ACCESS_CONTROL_SERVER}}
 artifacts-server={{GITLAB_PAGES_ARTIFACTS_SERVER_URL}}
+internal-gitlab-server=http://localhost:8181
+api-secret-key=/home/git/gitlab/.gitlab_pages_secret

assets/runtime/functions

@@ -752,6 +752,12 @@ gitlab_configure_secrets() {
     exec_as_git openssl rand -base64 -out "${workhorse_secret}" 32
     chmod 600 "${workhorse_secret}"
   fi
+
+  local pages_secret="${GITLAB_INSTALL_DIR}/.gitlab_pages_secret"
+  if [[ ! -f "${pages_secret}" ]]; then
+    exec_as_git openssl rand -base64 -out "${pages_secret}" 32
+    chmod 600 "${pages_secret}"
+  fi
 }
 
 gitlab_configure_sidekiq() {