host Linux server 4.15.0-22-generic #24-Ubuntu SMP Wed May 16 12:15:17 UTC 2018 x86_64 GNU/Linux
It took a while to figure out that changes in /etc/lxc/dnsmasq.conf will only be applied after having lxc-net restarted, or at least that is how it worked out on this box.
Perhaps that is logical/apparent to users with extensive networking knowledge but it might be less so for the less expirienced user. Having looked into the LXC online documentation I failed to find any such stipulation. Perhaps that could be improved.
In case the container’s ip is still not assigned as designated in dnsmasq.conf it might be worth flushing the dhcp pool cached by dnsmasq - echo "" > /var/lib/misc/dnsmasq.lxcbr0.leases and restart lxc-net afterwards