SSH reliability settings

* enable SSHD multiplexing, KeepAlive and relax throttling

Thank you for the hint:
https://forums.docker.com/t/docker-compose-through-ssh-failing-and-referring-to-docker-example-com/115165/18?u=ab77

       MaxStartups
               Specifies the maximum number of concurrent
               unauthenticated connections to the SSH daemon.
               Additional connections will be dropped until
               authentication succeeds or the LoginGraceTime expires for
               a connection.  The default is 10:30:100.

               Alternatively, random early drop can be enabled by
               specifying the three colon separated values
               start:rate:full (e.g. "10:30:60").  sshd(8) will refuse
               connection attempts with a probability of rate/100 (30%)
               if there are currently start (10) unauthenticated
               connections.  The probability increases linearly and all
               connection attempts are refused if the number of
               unauthenticated connections reaches full (60).

change-type: patch

.github/workflows/tests.yml

@@ -798,6 +798,7 @@ jobs:
                 && rm -f "\${tmphosts}" \
                 && getent hosts api.${{ matrix.subdomain }}.${{ matrix.dns_tld }} | grep 127.0.1.1
 
+              sshd -T
               service ssh restart
 
           # https://forums.docker.com/t/docker-compose-through-ssh-failing-and-referring-to-docker-example-com/115165/18