replace use of docker registry docker.io (hub.docker.com) with self-hosted r.xmox.nl

when setting up the docker organization account for mox it already felt off.
depending on such a party to serve binaries didn't feel great to begin with.
after clarifying online discussions of docker's vague announcement of removing
free team organizations it was clearly time to move off hub.docker.com.  best to
self-host.

r.xmox.nl is hosted with the new github.com/mjl-/vex.

pinging issue #3

docker-compose.yml

@@ -14,8 +14,9 @@
 version: '3.7'
 services:
   mox:
-    # Replace latest with the version you want to run.
-    image: docker.io/moxmail/mox:latest
+    # Replace "latest" with the version you want to run, see https://r.xmox.nl/repo/mox/.
+    # Include the @sha256:... digest to ensure you get the listed image.
+    image: r.xmox.nl/mox:latest
     environment:
       - MOX_DOCKER=yes # Quickstart won't try to write systemd service file.
     # Mox needs host networking because it needs access to the IPs of the