dockerfile: allow external dockerfile on remote context

[tonistiigi]

This allows dockerfile to be outside of the build context when building from a remote context. We check if the key for dockerfile local was sent with a request and if it was we use the dockerfile from the local instead of the one in the remote.

This enables implementing moby/moby#38254

@AkihiroSuda @thaJeztah

Signed-off-by: Tonis Tiigi [email protected]