Excellent - this is something I shall try out on a sandbox instance. I’ll see if I can report back if it works or not 
Understood. I am now pondering whether there should be a feature request for an option to export this key in backups? To me, losing all the sessions on a site would seem like a ‘very bad thing’ to happen as there are a few thousand accounts on the site. Practicing the migration has shown this problem, but I guess if the machine or Docker instance were lost for some reason, the key would be lost too and we’d be facing the same loss of sessions.
Some note of this ‘feature’ on the migration topic might be good too: Move your Discourse Instance to a Different Server