Unless you’re downloading a prebuilt LXC, you’d still have to do all the manual install yourself.
If you do download a prebuilt one, then you’ll need to do the updating yourself, like you would a normal application, including ensuring you keep dependencies up to date and all that.
Both have their pros and cons and I use each depending on what I’m doing (and basically all of my dockers are running in their own LXC containers, which I find to be the best of both worlds).
FWIW, I don’t download any prebuilt LXC anymore other than the base ‘Ubuntu’ or ‘Debian’ ones … the ones in ProxMox that have the prebuilt apps were a pain to update for me, especially since I had no idea how they were actually installed and most of the times they didn’t have package manager installations or curl installed and it was just way more trouble than it was worth.
ProxMox does now have a built in containerized docker implementation that will use an LXC and you can just provide it the docker package details, but, it’s still in beta and I don’t know that it’s ready to be depended on yet.
Sorry, not 100% sure what you mean “converting its spec”
If you mean take an existing docker and move it to a standard installation, that would depend on what all is needed. Some installations include a ton of other dockers with databases and such and you’d basically need to move them all independently and ensure the programs talk to each other properly.
For others, it’s be as simple as making sure the contents of your original docker data folder is in the right place when you launch the app and you’re done.