Give docker-manager its project on the shared Zitadel

Its OIDC login isn't implemented yet, but it's meant to use the same
Zitadel as Musify and Ping. Provision its Zitadel project and service
account now, so its future zitadel-init only has to create its apps.

Document that every project shares one Zitadel organization: one account
per person across all projects, with single sign-on between them.

compose.yml

@@ -278,7 +278,7 @@ services:
       PING_ZITADEL_PROJECT: Ping
       DOCKERMANAGER_DB_PASSWORD: ${DOCKERMANAGER_DB_PASSWORD:-}
       DOCKERMANAGER_RABBITMQ_PASSWORD: ${DOCKERMANAGER_RABBITMQ_PASSWORD:-}
-      DOCKERMANAGER_ZITADEL_PROJECT: ""
+      DOCKERMANAGER_ZITADEL_PROJECT: Docker Manager
     volumes:
       - ./provision:/provision:ro
       - ./zitadel/.output:/output