Allow heredocs to be defined anywhere on the instruction's first line

[rcjsuen]

# syntax=docker/dockerfile-upstream:master-labs
FROM alpine
COPY a.txt <<file b.txt <<file2 /destination/
abc
file
def
file2

$ DOCKER_BUILDKIT=1 docker build --no-cache .
[+] Building 1.2s (13/13) FINISHED                                          
 => [internal] load build definition from Dockerfile                   0.0s
 => => transferring dockerfile: 31B                                    0.0s
 => [internal] load .dockerignore                                      0.0s
 => => transferring context: 2B                                        0.0s
 => resolve image config for docker.io/docker/dockerfile-upstream:mas  0.1s
 => CACHED docker-image://docker.io/docker/dockerfile-upstream:master  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
 => CACHED [internal] preparing inline document                        0.0s
 => [internal] load build context                                      0.0s
 => => transferring context: 52B                                       0.0s
 => [1/2] FROM docker.io/library/alpine@sha256:eb3e4e175ba6d212ba1d6e  0.3s
 => => resolve docker.io/library/alpine@sha256:eb3e4e175ba6d212ba1d6e  0.0s
 => => sha256:021b3423115ff662225e83d7e2606475217de7b 1.47kB / 1.47kB  0.0s
 => => sha256:29291e31a76a7e560b9b7ad3cada56e8c18d50a 2.81MB / 2.81MB  0.1s
 => => sha256:eb3e4e175ba6d212ba1d6e04fc0782916c08e1c 1.64kB / 1.64kB  0.0s
 => => sha256:be9bdc0ef8e96dbc428dc189b31e2e3b05523d96d12 528B / 528B  0.0s
 => => extracting sha256:29291e31a76a7e560b9b7ad3cada56e8c18d50a96cca  0.2s
 => CACHED [internal] preparing inline document                        0.0s
 => [2/2] COPY a.txt <<file b.txt <<file2 /destination/                0.2s
 => exporting to image                                                 0.0s
 => => exporting layers                                                0.0s
 => => writing image sha256:278b87bb3d532c3d659462a84487ae4713ca06f39  0.0s

[rcjsuen]

This is only implemented for ADD, COPY, RUN, and their corresponding ONBUILD versions. We'll keep an eye on how things evolve in BuildKit and add support for other instructions as they come up.