Create a diagnostic if a duplicate instruction flag has been specified

[rcjsuen]

The following two Dockerfiles will not build.

FROM scratch
HEALTHCHECK --interval=10 --interval=10 CMD ls	

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : HEALTHCHECK --interval=10 --interval=10 CMD ls
Duplicate flag specified: interval

FROM scratch AS setup
COPY --from=setup --from=setup . .

$ docker build .
Sending build context to Docker daemon   59.6MB
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch AS setup
 --->
Step 2/2 : COPY --from=setup --from=setup . .
Duplicate flag specified: from

The following Dockerfile will build as the engine does not validate flags for a HEALTHCHECK NONE instruction.

FROM scratch
HEALTHCHECK --interval=10 --interval=10 NONE

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : HEALTHCHECK --interval=10 --interval=10 NONE
 ---> Running in f3e084086096
 ---> f5df294db6b4
Removing intermediate container f3e084086096
Successfully built f5df294db6b4