LetsCloud´s not a domain name registrar, but does provide a DNS hosting service. You need to purchase a domain name from a domain name registrar.
LetsCloud offers free DNS hosting for customers' domains.
Regarding security, containers share the kernel, some components of the system and also have root access. If compared to a virtual machine, containers are more exposed to vulnerabilities. But there are resources to increase containers’ security, such as creating isolation layers between applications and other components.