Let's remain sticky to Redis 4.3.0

Some days ago Redis 5.0.0 was released and it includes some
changes that require verification in core functionality and
associated unit tests (now failing). To be handled by:

https://tracker.moodle.org/browse/MDL-66139

Once that issue is fixed... surely we can unpin this.

Ciao :-)

Author: stronk7

root/tmp/setup/php-extensions.sh

@@ -59,7 +59,7 @@ docker-php-ext-configure ldap --with-libdir=lib/x86_64-linux-gnu/
 docker-php-ext-install -j$(nproc) ldap
 
 # SOLR, Memcached, Redis, APCu, igbinary.
-pecl install solr memcached mongodb redis apcu igbinary
+pecl install solr memcached mongodb redis-4.3.0 apcu igbinary
 docker-php-ext-enable solr memcached mongodb redis apcu igbinary
 
 echo 'apc.enable_cli = On' >> /usr/local/etc/php/conf.d/docker-php-ext-apcu.ini