Support new --checksum flag for ADD instructions

[rcjsuen]

The feature is not supported out-of-the-box but can be enabled with #syntax=docker/dockerfile:1

FROM alpine
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /

$ docker build .
[+] Building 0.2s (3/3) FINISHED                                                                docker:default
 => [internal] load .dockerignore                                                                         0.0s
 => => transferring context: 2B                                                                           0.0s
 => [internal] load build definition from Dockerfile                                                      0.0s
 => => transferring dockerfile: 216B                                                                      0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                          0.2s
Dockerfile:2
--------------------
   1 |     FROM alpine
   2 | >>> ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /
   3 |
--------------------
ERROR: failed to solve: instruction 'ADD --checksum=<CHECKSUM>' requires the labs channel

#syntax=docker/dockerfile:1
FROM alpine
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /

Line: 3
ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d https://mirrors.edge.kernel.org/pub/linux/kernel/Historic/linux-0.01.tar.gz /
    ^^^^^^^^^^
Error: Unknown flag: checksum