How to I add a GUI to an LXD Container

[Q-nap]

I've used Docker for a while now and found its pre built images useful. However, after just spending some time rebuilding them because they can't easily be upgraded I looked at LXD instead. I've now replaced some of my Docker containers with LXD ones that I have built myself and know what went in them. Pi-Hole for example now runs in an LXD container and I can now upgrade it with a single command line rather than rebuilding a new docker image from scratch. I can convert any of my Docker images that have a Web I/F. You just connect up using http and away you go. Easy and convenient. imo LXD is much better than Docker if you can build your own images.

However, I can't convert the ones that don't have a web I/F. For example Thunderbird EMail client. I can install it fine and trigger via terminal it but can't access it remotely.

I've searched online and all the web sites explain about setting up LXC/D for a GUI but they use LXC commands to do so. As QNAP doesn't allow that in its very limited toolset for LXC/D I can't replicate it. I could set up a LXC/D system and build it there and import it to the QNAP but I'd rather not do that.

Anyone know how to create an LXD image that can be connected to via http or something else and act as a GUI so I can user Thunderbird? It is possible because there are Docker images there that can do it. Any ideas?