Dockerfile: improve error message when using instructions before first "FROM"

Relates to https://github.com/docker/buildx/issues/426#issuecomment-819457197

The Dockerfile syntax only allows ARG to be specified before the first FROM. If a user accidentally puts any other Dockerfile instruction before the first FROM, a confusing error message is produced.

For example:

docker build -<<EOF
MAINTAINER foobar
FROM busybox
EOF

Produces:

[+] Building 0.1s (2/2) FINISHED
 => [internal] load build definition from Dockerfile                        0.0s
 => => transferring dockerfile: 70B                                         0.0s
 => [internal] load .dockerignore                                           0.0s
 => => transferring context: 2B                                             0.0s
failed to solve with frontend dockerfile.v0: failed to create LLB definition: no build stage in current context

Note that the error is equally "bad" on the classic builder (but less "noise");

DOCKER_BUILDKIT=0 docker build -<<EOF
MAINTAINER foobar
FROM busybox
EOF

Sending build context to Docker daemon  2.048kB
Error response from daemon: no build stage in current context

This error message could be improved to be more descriptive:

• mention that an instruction was used before the first FROM, but not allowed
• include the instruction in the error? (probably not needed if the new "rich" errors are used (with buildx))

More in general, perhaps we should include a link to the documentation, which could be a /go/ redirect (as permalink), e.g. https://docs.docker.com/go/dockerfile/ or https://docs.docker.com/go/dockerfile-syntax/

Reproduce the BuildKit and classic-builder examples in the issue, then trace validation of Dockerfile instructions before the first FROM. Done means an invalid pre-FROM instruction produces a descriptive error that explains the ordering rule, with the documentation-link decision resolved.