RUN instructions with a single flag should raise an error

[rcjsuen]

FROM scratch
RUN --x=y

$ docker build .
Sending build context to Docker daemon  38.15MB
Error response from daemon: Dockerfile parse error line 2: Unknown flag: x

$ docker run -v `pwd`/Dockerfile:/Dockerfile rcjsuen/dockerfile-utils lint /Dockerfile
$ 

The error is a bit difficult to mirror given that RUN instructions have various experimental flags in BuildKit at the moment. We could at least flag this instruction as missing an argument as it does.