laravel-horizon: Install 'sockets' without install 'AMQP` (#2211)

* Add LARAVEL_HORIZON_INSTALL_SOCKETS variable. Issue #2209
* Get LARAVEL_HORIZON_INSTALL_SOCKETS from .env. Issue #2209
* Get INSTALL_SOCKETS variable and install 'sockets' ext. Issue #2209

laravel-horizon/Dockerfile

@@ -33,8 +33,8 @@ RUN if [ ${INSTALL_BCMATH} = true ]; then \
   ;fi
 
 #Install Sockets package:
-ARG INSTALL_AMQP=false
-RUN if [ ${INSTALL_AMQP} = true ]; then \
+ARG INSTALL_SOCKETS=false
+RUN if [ ${INSTALL_SOCKETS} = true ]; then \
   docker-php-ext-install sockets \
   ;fi