Point PHP openssl validators at /etc/ssl/certs

The default location for PHP's OpenSSL cert validation is /usr/lib/ssl,
which is not available in our containers: instead, we bind-mount a
system-wide /etc/ssl directory. This change should make PHP look in
that directory instead.

conf/php/7.4/conf.d/50-container-openssl.conf

+[openssl]
+openssl.capath=/etc/ssl/certs