dockerfile: removed date tag from main `apt-get` line
dockerfile: removed date tag from main `apt-get` line

opendoor-labs/docker-squid-s3,korczis/docker-squid-rubygems-proxy,maqsoodumt/docker-squid,linearregression/docker-squid-s3,sameersbn/docker-squid,Optibus/docker-squid,zultron/docker-provisioning,korczis/docker-squid-rubygems-proxy,boliu83/docker-squid