Create a diagnostic if HEALTHCHECK's interval, start-period, and timeout flags are invalid

[rcjsuen]

If you try to enter in characters for the interval, start-period, or timeout flag of a HEALTHCHECK instruction, the engine will throw an error. We should do the same and create a diagnostic for this case.

FROM scratch AS setup
HEALTHCHECK --start-period=a CMD ls

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch AS setup
 --->
Step 2/2 : HEALTHCHECK --start-period=a CMD ls
time: invalid duration a