cli with buildkit will ignore dockerignore's check

Description
docker CLI with builkit enabled will ignore .dockerignore file's read errors.

Steps to reproduce the issue:

(MoeLove) ➜  x cat Dockerfile 
FROM scratch

COPY foo foo
COPY bar bar
(MoeLove) ➜  x ls 
bar  Dockerfile  foo
(MoeLove) ➜  x cat Dockerfile 
FROM scratch

COPY foo foo
COPY bar bar
(MoeLove) ➜  x ls -al .dockerignore 
----------. 1 tao tao 4 6月  12 02:10 .dockerignore
(MoeLove) ➜  x cat .dockerignore           
cat: .dockerignore: Permission denied
(MoeLove) ➜  x whoami
tao
(MoeLove) ➜  x docker build --no-cache -t local/ignore .
[+] Building 0.1s (6/6) FINISHED                                                                                 
 => [internal] load .dockerignore                                                                           0.0s
 => => transferring context: 91B                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                        0.0s
 => => transferring dockerfile: 95B                                                                         0.0s
 => [internal] load build context                                                                           0.0s
 => => transferring context: 162B                                                                           0.0s
 => [1/2] COPY foo foo                                                                                      0.0s
 => [2/2] COPY bar bar                                                                                      0.0s
 => exporting to image                                                                                      0.0s
 => => exporting layers                                                                                     0.0s
 => => writing image sha256:5ae5f1e9c56ee852af6a47c0aa4fe78fba3853e720171047617e8b022f06f456                0.0s
 => => naming to docker.io/local/ignore                                                                     0.0s

Describe the results you received:

Build image successfully.

Describe the results you expected:

Build image failed.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

(MoeLove) ➜  x docker version                                                                                    
Client: Docker Engine - Community
 Version:           0.0.0-20190607210243-b93f571b01
 API version:       1.40   
 Go version:        go1.12.5 
 Git commit:        b93f571b01
 Built:             Sat Jun  8 07:31:22 2019
 OS/Arch:           linux/amd64
 Experimental:      true

Output of docker info:

(paste your output here)

Additional environment details (AWS, VirtualBox, physical, etc.):
physical

Reproduce the issue with the mentioned Dockerfile, unreadable .dockerignore, and docker build command while BuildKit is enabled. Start at the CLI build entry point and trace how the build context reports .dockerignore read errors. Done means the same permission failure causes the build to fail, with a regression test covering it.