Eu apenas tentei isso:
user@mybox:~$ lxc launch ubuntu:16.04 test
mybox:~$ lxc exec test /bin/bash
root@test:~# apt install php7.0-fpm php7.0-curl
root@test:~# php7.0 -i | grep -i curl
/etc/php/7.0/cli/conf.d/20-curl.ini,
curl
cURL support => enabled
cURL Information => 7.47.0
verifique se há um link simbólico de /etc/php/7.0/*/conf.d/20-curl.ini
para /etc/php/7.0/mods-available/curl.ini