The provider currently attempts to parse your Dockerfile in case there
are any obviously broken syntax errors.
However, Dockerfiles can use custom syntaxes which are implemented as
their own images. Buildkit detects `# syntax=` directives, pulls the
image, and uses it to parse.
It doesn't make sense for us to attempt to replicate that behavior, so
when we detect a custom syntax we will simply disable this validation.
If there are syntax errors the user will discover them after the build
is attempted.
Fixes https://github.com/pulumi/pulumi-docker-build/issues/300
