Flag STOPSIGNAL with $ argument as an error

[rcjsuen]

If we run the CLI linter on the file below, we won't get any errors. However, the Docker builder will not build the image as $ is not a valid STOPSIGNAL argument so the linter should be updated to handle this case.

FROM scratch
STOPSIGNAL $

$ docker build .
Sending build context to Docker daemon  1.871MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : STOPSIGNAL $
Invalid signal: $