Publish diagnostic if a COPY's --from flag is invalid

[rcjsuen]

FROM scratch
COPY --from= . .

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM scratch
 --->
Step 2/2 : COPY --from= . .
invalid from flag value : invalid reference format

In the Dockerfile above, we specify the --from= flag to the CMD instruction but don't actually specify any values. The Docker engine will warn about this case so we should publish a diagnostic that achieves the same thing.