Flag double hyphens as an invalid duration

[rcjsuen]

If there are two hyphens in a duration flag, it should be flagged as an error.

FROM alpine
HEALTHCHECK --timeout=--5s CMD ls

$ dockerfile-utils lint Dockerfile
$

$ docker build .
Sending build context to Docker daemon  1.696MB
Error response from daemon: Dockerfile parse error line 2: time: invalid duration --5s