Did not work, I'll define a configmap instead

kubernetes/pihole/pihole/app/headers.yaml

@@ -15,21 +15,4 @@ spec:
  contentSecurityPolicy: "default-src 'none'; script-src 'self' 'unsafe-inline' 'unsafe-eval' https:; style-src 'self' 'unsafe-inline' https:; img-src 'self' data: https:; font-src 'self' https: data:; connect-src 'self' https:; frame-src 'self' https:; media-src 'self' https:; object-src 'none'; frame-ancestors 'self'; base-uri 'self'; form-action 'self';"
  customFrameOptionsValue: SAMEORIGIN
  customRequestHeaders:
- X-Forwarded-Proto: https
- accessControlAllowOriginList:
- - "https://10.42.3.64"
- - "https://pihole.local.solonsstuff.com"
- - "https://pi.hole"
- - "https://localhost"
- - "https://pihole-7d8d5555cd-2jpk2"
- accessControlAllowMethods:
- - "GET"
- - "POST"
- - "PUT"
- - "DELETE"
- - "OPTIONS"
- accessControlAllowHeaders:
- - "Content-Type"
- - "Authorization"
- - "X-Requested-With"
- accessControlAllowCredentials: true
+ X-Forwarded-Proto: https