Composer 2 in images

[marijnotte]

Composer 2 has been released stable on October 24 (https://blog.packagist.com/composer-2-0-is-now-available/)

We would like to have this new version in the images

(rebuilding the images might be enough to upgrade)

[mbrodala]

Actually there should be an option to not upgrade to Composer 2.x since this can lead to unexpected issues until everything has been prepared properly.

[mbrodala]

See #223 for a hotfix to enforce Composer 1.x for now. A Composer version selection should be done separately IMO.

[moufmouf]

We are going to create a v4 of the image with Composer 2 and keep v3 with Composer 1.
See #224