feat: Use ghcr.io for custom images instead of docker.io

[markometcalfe]

Description

This change makes it so our custom containers use the images hosted on Github here.
The benefit of doing this is cost savings. Public packages on Github are entirely free, whereas for Docker hub we have to have a paid subscription for hosting images.

Testing Instructions

1. Check out this pull request
2. Start all of the containers via tdocker pull && tdocker up -d
3. Check that running tpull runs fine
4. For each of the PHP containers (there is a lot!):
   1. Ensure a site can be visited as expected
   2. Ensure PHPUnit and Behat tests run as expected
   3. Ensure you can run PHPUnit tests from PHPStorm as before
   4. Ensure XDebug breakpoints get triggered in PHPStorm as before
5. For each of the mssql2017, mssql2019, mssql2022 containers:
   1. Ensure you can change your site config.php to use that mssql version and you can install a site fine

Checklist

• Does what the author says it will do
• Testing instructions are provided
• Commit messages make sense and follow the conventional commit standard
• No identified security issues
• No identified maintenance issues
• Any third-party libraries/dependencies use the MIT or Apache 2.0 license
• Changes made are backwards compatible and will not break existing setups
• Changes to scripts in the bin/ directory run correctly on both MacOS and WSL
• Changes to containers can be built locally sucessfully (e.g. via tbuild container && tup container)
• Containers/images are compatible with both AMD64 (Windows) and ARM64 (MacOS)
• Changes made to config.php are compatible with our oldest supported Totara version, our newest Totara version, and Moodle

[TotaraAdmin]

🎉 This PR is included in version 1.21.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀