run apt-get update before attempting to install php-redis on workspace container

Fixes E: Failed to fetch http://ppa.launchpad.net/ondrej/php/ubuntu/pool/main/p/php-redis/php-redis_4.2.0-1+ubuntu16.04.1+deb.sury.org+1_amd64.deb  404  Not Found

workspace/Dockerfile

@@ -414,6 +414,7 @@ RUN if [ ${INSTALL_AMQP} = true ]; then \
 ARG INSTALL_PHPREDIS=false
 
 RUN if [ ${INSTALL_PHPREDIS} = true ]; then \
+    apt-get update -yqq && \
     apt-get install -yqq php-redis \
 ;fi