Caddyfile

# ...

# Matches requests for OIDC routes
@oidc expression path('/oidc/*')

route {
    # ...
    reverse_proxy @oidc http://{$OIDC_UPSTREAM}
}

docker-compose.yaml

version: "3.8"

services:
  # ...

  php:
    # ...
    environment:
      # ...
      OIDC_UPSTREAM: keycloak:8080

  keycloak:
    image: bitnami/keycloak:21-debian-11
    environment:
      # ...
      # Must finish with a trailing slash (https://github.com/bitnami/charts/issues/10885#issuecomment-1414279144)
      KEYCLOAK_HTTP_RELATIVE_PATH: /oidc/
      # https://www.keycloak.org/server/hostname
      KC_HOSTNAME_URL: https://${SERVER_NAME:-localhost}/oidc/

Merci pour le retour !