You’re missing proxy headers that configure the nginx proxy behaviour to do what you want, it doesn’t do it out of the box.
Some ref material includes the standard nginx template if you use Ghost CLI:
And our docs for if you were proxying to Ghost(Pro)
Which includes an nginx template:
There’s a bunch of different settings in there that are worth looking up, but specifically you’d want to be setting X-Forwarded-For, X-Forwarded-Host and probably X-Forwarded-Proto to pass the orignal settings through to Ghost.