reverse tunnel needs an explanation on reverse tunnel key creation

[simmonsm]

When setting-up the autossh tunnel service file a reverse_tunnel_key is referred to in the following block:

ExecStart=/usr/bin/autossh -M 0 -4 -N
-o "ServerAliveInterval=30"
-o "ServerAliveCountMax=3"
-o "ExitOnForwardFailure=yes"
-o "StrictHostKeyChecking=yes"
-i /home/tunnel-user/.ssh/reverse_tunnel_key
-R 127.0.0.1:2222:localhost:22
tunnel@203.0.113.10

The steps for the creation of this key should be supplied for those who are less familiar with the subject.