With this patch the following become true even with buildkit enabled:
1. `docker build -q .` only outputs the created image's sha256 ID.
2. `docker build -q .` outputs as if no `-q` was specified, if error occurred
3. `docker build . &> out` outputs JSON (instead of TTY characters)
Signed-off-by: Tibor Vass <tibor@docker.com>