Optimized OIDC integration for mailu

roles/docker-mastodon/templates/env.j2

@@ -64,12 +64,14 @@ OIDC_DISPLAY_NAME="{{oidc.button_text}}"
 OIDC_ISSUER={{oidc.client.issuer_url}}
 OIDC_DISCOVERY=true
 OIDC_SCOPE="openid,profile,email"
-OIDC_UID_FIELD=preferred_username                       # @see https://stackoverflow.com/questions/72108087/how-to-set-the-username-of-mastodon-by-log-in-via-keycloak
+# @see https://stackoverflow.com/questions/72108087/how-to-set-the-username-of-mastodon-by-log-in-via-keycloak
+OIDC_UID_FIELD={{oidc.attributes.username}}
 OIDC_CLIENT_ID={{oidc.client.id}}
 OIDC_REDIRECT_URI=https://{{domains[application_id]}}/auth/auth/openid_connect/callback
 OIDC_SECURITY_ASSUME_EMAIL_IS_VERIFIED=true
 OIDC_CLIENT_SECRET={{oidc.client.secret}}
-OMNIAUTH_ONLY=true                                      # uncomment to only use OIDC for login / registration buttons
+# uncomment to only use OIDC for login / registration buttons
+OMNIAUTH_ONLY=true
 ALLOW_UNSAFE_AUTH_PROVIDER_REATTACH=true
 ONE_CLICK_SSO_LOGIN=true
 {% endif %}