Create a diagnostic if HEALTHCHECK's retries flag is not a positive integer

[rcjsuen]

The value for the retries flag must be a positive integer. We should create a diagnostic for this error.

FROM alpine
HEALTHCHECK --retries=0 CMD ls

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM alpine
 ---> 7328f6f8b418
Step 2/2 : HEALTHCHECK --retries=0 CMD ls
--retries must be at least 1 (not 0)

FROM alpine
HEALTHCHECK --retries=-1 CMD ls

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM alpine
 ---> 7328f6f8b418
Step 2/2 : HEALTHCHECK --retries=-1 CMD ls
--retries must be at least 1 (not -1)