This allows us to be backwards compatible when executing git:from-image.
The nice thing about this pattern is we can also extend it to other parts of the deployment as well, such asn nginx-vhosts fetching
The previous method was a bit more difficult for users to interact with as they needed to ensure the file was at a certain path in the built image. This change divorces the file from the built artifact, better aligning the nginx.conf.sigil handling with the rest of Dokku's monorepo support.
A side effect of this is now building proxy configurations does not require access to the built image.
Closes#5207
