Add composer to php images

[markmetcalfe]

This PR adds composer to the PHP 7.3+ containers, and also fixes a build issue with PHP 8.4.

To test this:

1. Check out this PR
2. Run:

   for v in php-7.3 php-7.4 php-8.0 php-8.1 php-8.2 php-8.3 php-8.4; do
       tbuild "$v" && tup "$v" && texec "$v" composer --version
   done

   and make sure there are no errors

[LinnyTheDuck]

Tested and working, LGTM