Publish diagnostic if HEALTHCHECK has flags but no arguments

[rcjsuen]

The validator currently considers a flag as an argument so it doesn't warn about HEALTHCHECK instructions that don't specify a CMD or a NONE. If a HEALTHCHECK instruction has flags but no arguments, a diagnostic should be published.

FROM alpine
HEALTHCHECK --retries=3

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM alpine
 ---> 7328f6f8b418
Step 2/2 : HEALTHCHECK --retries=3
HEALTHCHECK requires at least one argument