Move default to Debian Buster and PHP 7.4?

[wmortada]

This follows from #68 and a suggestion from Mikey that we should be using debian buster. Should we be looking to make the jump? I had issues when I tried previously - see #60.

[wmortada]

I guess all this really means is making the PHP 7.4 image the default but we need to fix #60 first.

[wmortada]

#60 is no longer an issue for me.

[michaelmcandrew]

Just trying this out...