RUN instruction does not need any arguments

[rcjsuen]

Seems like you can have zero instructions for RUN when using BuildKit so we should add support for this.

FROM alpine:latest
RUN --mount=type=cache,target=/var/cache/foo,uid=1,gid=1
RUN

$ DOCKER_BUILDKIT=1 docker build --no-cache .
[+] Building 1.6s (8/8) FINISHED                                                                                           
 => [internal] load build definition from Dockerfile                                                                  0.0s
 => => transferring dockerfile: 116B                                                                                  0.0s
 => [internal] load .dockerignore                                                                                     0.0s
 => => transferring context: 2B                                                                                       0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                                      0.1s
 => CACHED [stage-0 1/3] FROM docker.io/library/alpine:latest@sha256:e1c082e3d3c45cccac829840a25941e679c25d438cc8412  0.0s
 => CACHED [internal] settings cache mount permissions                                                                0.0s
 => [stage-0 2/3] RUN --mount=type=cache,target=/var/cache/foo,uid=1,gid=1                                            0.8s
 => [stage-0 3/3] RUN                                                                                                 0.7s
 => exporting to image                                                                                                0.0s
 => => exporting layers                                                                                               0.0s
 => => writing image sha256:e3ee95c220624a5742c40a4fd98a3161d7b50a9c582bf3b1a2387a4679aebe54                          0.0s

$ node bin/dockerfile-utils lint
Line: 2
RUN --mount=type=cache,target=/var/cache/foo,uid=1,gid=1
^^^
Error: Instruction has no arguments

Line: 3
RUN
^^^
Error: Instruction has no arguments