Add support for heredocs in ADD and COPY

[rcjsuen]

# syntax=docker/dockerfile-upstream:master-labs
FROM alpine
COPY <<eot /app/out.txt
  hello
eot

$ DOCKER_BUILDKIT=1 docker build . --no-cache -t test
[+] Building 1.0s (11/11) FINISHED                                              
 => [internal] load build definition from Dockerfile                       0.0s
 => => transferring dockerfile: 132B                                       0.0s
 => [internal] load .dockerignore                                          0.0s
 => => transferring context: 2B                                            0.0s
 => resolve image config for docker.io/docker/dockerfile-upstream:master-  0.2s
 => CACHED docker-image://docker.io/docker/dockerfile-upstream:master-lab  0.0s
 => [internal] load build definition from Dockerfile                       0.0s
 => [internal] load .dockerignore                                          0.0s
 => [internal] load metadata for docker.io/library/alpine:latest           0.1s
 => [internal] preparing inline document                                   0.0s
 => CACHED [1/2] FROM docker.io/library/alpine@sha256:e1c082e3d3c45cccac8  0.0s
 => [2/2] COPY <<eot /app/out.txt                                          0.1s
 => exporting to image                                                     0.0s
 => => exporting layers                                                    0.0s
 => => writing image sha256:ea6a40bd045d6a06e5eaf39ec34b1dec85d9279a23352  0.0s
 => => naming to docker.io/library/test                                    0.0s

const utils = require("dockerfile-utils")
const problems = utils.validate("FROM alpine\nCOPY <<eot /app/out.txt\n  hello\neot");
console.log(problems.length);

1

The Dockerfile above is valid with the new heredoc syntax so we should not be reporting any errors.