PSR conflict with Symfony

[pvdleek]

Hi, I ran into an issue today that the PSR extension conflicts with the Symfony cache component. I therefore had to remove PSR, but as there is no "docker-php-ext-disable" (and it will not be there, see https://github.com/docker-library/php/issues/220#issuecomment-353677238) I tried "pecl uninstall psr". Unfortunately that created another issue:

PHP Warning:  PHP Startup: Unable to load dynamic library 'psr' (tried: /usr/local/lib/php/extensions/no-debug-non-zts-20230831/psr (Error loading shared library /usr/local/lib/php/extensions/no-debug-non-zts-20230831/psr: No such file or directory), /usr/local/lib/php/extensions/no-debug-non-zts-20230831/psr.so (Error loading shared library /usr/local/lib/php/extensions/no-debug-non-zts-20230831/psr.so: No such file or directory)) in Unknown on line 0

I could of course create a PR for the removal of PSR, but it might be more interesting to have a ENV variable control the installation of PSR (and maybe other extensions).

How do you feel about this?