Add support for --link flag in ADD

[rcjsuen]

FROM scratch
ADD --link . .

$ DOCKER_BUILDKIT=1 docker build .
[+] Building 13.7s (9/9) FINISHED                                                                                                                   
 => [internal] load build definition from Dockerfile                                                                                           0.0s
 => => transferring dockerfile: 95B                                                                                                            0.0s
 => [internal] load .dockerignore                                                                                                              0.1s
 => => transferring context: 2B                                                                                                                0.0s
 => resolve image config for docker.io/docker/dockerfile:1.4                                                                                   1.1s
 => docker-image://docker.io/docker/dockerfile:1.4@sha256:91f386bc3ae6cd5585fbd02f811e295b4a7020c23c7691d686830bf6233e91ad                     4.9s
 => => resolve docker.io/docker/dockerfile:1.4@sha256:91f386bc3ae6cd5585fbd02f811e295b4a7020c23c7691d686830bf6233e91ad                         0.0s
 => => sha256:91f386bc3ae6cd5585fbd02f811e295b4a7020c23c7691d686830bf6233e91ad 2.00kB / 2.00kB                                                 0.0s
 => => sha256:7c651b7122e7366e15d4ea5327b27111f6862d9b9896383a4a695d3190f3909d 528B / 528B                                                     0.0s
 => => sha256:edddd0dce574bd03c542d84013e16caf779de32ce2a1c5b402a0bc7405f85b20 2.37kB / 2.37kB                                                 0.0s
 => => sha256:0f821387874bc19cdfd45922f5ff075fd256356bf3f2baba7fb109d2b3a64bbd 9.93MB / 9.93MB                                                 0.2s
 => => extracting sha256:0f821387874bc19cdfd45922f5ff075fd256356bf3f2baba7fb109d2b3a64bbd                                                      4.6s
 => [internal] load .dockerignore                                                                                                              0.0s
 => [internal] load build definition from Dockerfile                                                                                           0.0s
 => [internal] load build context                                                                                                              1.8s
 => => transferring context: 46.89MB                                                                                                           1.8s
 => [1/1] ADD --link . .                                                                                                                       0.3s
 => exporting to image                                                                                                                         0.4s
 => => exporting layers                                                                                                                        0.4s
 => => writing image sha256:fe01f7822cbaf8c91386359d46b82789c9a9b9fb54e3cd09c58f8eaf65b0f626                                                   0.0s

const content = `FROM scratch
ADD --link . .`;
const dockerfileUtils = require("dockerfile-utils");
console.log(dockerfileUtils.validate(content).length);

1