I assume you mean Nginx Proxy Manager? I'm surprised that you would even run that on a desktop with a GUI, seems far more fit for a headless system. Of course, nothing stops you - it's your system.
As a general note I'd recommend docker CLI / compose, most applications will assume you're using that and have instructions tailored for it (which is helpful if you're new to docker).
To be honest I didn't even know docker had a desktop app for Linux, I've only seen folks use it on Windows and macOS.
And if you're not sure whether the cache has been fully flushed, run
syncand it'll return once it's done!