Do not hardcode "ubuntu" for the Docker APT key URL

Well, `ubuntu` or `debian`, the same key is served right now,
so it doesn't really matter.

This seems cleaner and less prone to breakage though.

roles/matrix-base/tasks/server_base/setup_debian.yml

@@ -11,7 +11,7 @@
 
 - name: Ensure Docker's APT key is trusted
   apt_key:
-    url: https://download.docker.com/linux/ubuntu/gpg
+    url: "https://download.docker.com/linux/{{ ansible_distribution|lower }}/gpg"
     id: 9DC858229FC7DD38854AE2D88D81803C0EBFCD88
     state: present
   register: add_repository_key