Add proxying to github for non-existing local assets

[andrey-yantsen]

In the existing implementation, you need to cache all the possible assets locally, or you won't be able to use them. This PR adds a way for end-users to use locally cached assets and, at the same time, enjoy an occasional non-cached asset.

I'm unsure if including this change in the main docker image is worth it: adding a new docker tag for non-security cautious people/environments where this will be enabled might be a better idea. Or maybe setting up an nginx server on a different port just for that. Please advise how you would like to proceed here.

I'm using netboot.xyz in a tiny just-for-fun homelab environment, and I very much enjoy having a few most-used assets in the local cache and still being able to boot any other available thing :)