regression in master-experimental gateway frontends with older daemons

[scaytrase]

Given a build described here

https://github.com/scaytrase/docker-experimental-syntax-issue

I run make build and receive the following error

 > [1/2] RUN cat /home/www-data/result:
------
failed to solve with frontend dockerfile.v0: failed to build LLB: executor failed running [/bin/sh -c cat /home/www-data/result]: unable to find user www-data: no matching entries in passwd fi
le
make: *** [build-usage] Error 1

(could be any command, even RUN echo "" fails the same way)

Rolling back syntax to 20191217-secrets. Any syntax later than this fails with the same error

https://github.com/scaytrase/docker-experimental-syntax-issue/blob/fixed/intermediate/Dockerfile#L1

Makes the build pass and work perfect.

[tonistiigi]

@hinshun Seems that this broke with #1269 . To test build any dockerfile with #syntax=docker/dockerfile-upstream/master-experimental with Docker 19.03, then run inspect on the resulting image and look at the empty layers in the rootfs field. PTAL

[hinshun]

@tonistiigi Looking into this