Howard Miller

My first thought would be to run 'show processlist' in the MySQL client (assuming you're using MySQL) while it's stuck. Is the database doing anything?

You can also run top/htop and see what the server is doing - is the php script consuming processor time and memory?

Noted - thank you.

From what I (now) understand, I don't think it's something useful for me (I'm happy with Docker) but I may have a look all the same.

It's not something I have done but - especially as you have tested it - I can't see why it shouldn't work.

I have absolutely no idea what you are asking. You're going to have to explain the configuration of your sites in rather more detail to get an answer.

But... if they are three completely separate Moodle installations, then I can't see why not.

You can create config.php "manually". Copy config-dist.php to config.php, open config.php in an editor and add your configuration changes as required (the file is very well commented). The you can run upgrade and it will just use your supplied config.php.