This means PHP 7.3->7.4 and we can use the Debian's own PHP packages

It's usually the right ID, so avoid any errors in that case.

Symfony prints a warning that it is recommended otherwise

While we directed the docker image to translate the URLs to https anyway
this can be confusing since git behaves differently in docker vs outside.

Also in the meantime we have made all repos public, so there is no auth needed
for the repos and the extra work required using https no longer applies.

We have switched to composer 2 everywhere and while 1.x should still work
we don't test with it. So install composer 2 in the docker image.

* Create the user in the image with the same UID than the host user.
  This makes it easier to share files between the host and the container.
* Install the PHP dependencies right away, so the user doesn't have to
  know anything about composer at the beginning.