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

[rcjsuen]

If the retries flag of a HEALTHCHECK instruction is not an integer, the engine will throw a parsing error. We should do the same and create a diagnostic for this case.

FROM scratch AS setup
HEALTHCHECK --retries=a CMD ls

$ docker  build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch AS setup
 --->
Step 2/2 : HEALTHCHECK --retries=a CMD ls
strconv.ParseInt: parsing "a": invalid syntax

[rcjsuen]

Decimals are not allowed.

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM busybox
 ---> d27c8abadd3d
Step 2/2 : HEALTHCHECK --retries=1.0 cmd ls
strconv.ParseInt: parsing "1.0": invalid syntax

Leading zeroes however are okay.

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM busybox
 ---> d27c8abadd3d
Step 2/2 : HEALTHCHECK --retries=01 cmd ls
 ---> Running in 5958aa37e488
 ---> d0ce91003585
Removing intermediate container 5958aa37e488
Successfully built d0ce91003585