The key in debian-docker-updates should have been updated with the
prior change (d6b47faf240f5b6b118d5ebfef2beb17b24866e2) ... but the
Release.gpg file is actually signed by the F273FCD8 subkey of
0EBFCD88. So update to that.
Also, things have changed such that it's "stable", "edge" and "test".
Assume main->stable
A manual run with this seems to have mirrored correctly.
Change-Id: I6d096957495f03f47b5ab8f25664e83be18ba4f5
f6f641d965