Update Dockerfile (laradock#2147)

Solve issue laradock#2146

Author: vlauciani

laravel-horizon/Dockerfile

@@ -20,7 +20,8 @@ RUN apk --update add wget \
   autoconf \
   cyrus-sasl-dev \
   libgsasl-dev \
-  supervisor
+  supervisor \
+  procps
 
 RUN docker-php-ext-install mysqli mbstring pdo pdo_mysql tokenizer xml pcntl
 RUN pecl channel-update pecl.php.net && pecl install memcached mcrypt-1.0.1 && docker-php-ext-enable memcached