Create diagnostic for warning about unknown flags

[rcjsuen]

FROM alpine AS x
FROM alpine as y
COPY --FROM=x . .

Trying to build the above Dockerfile will generate the following error. We should create a diagnostic to recognize this error.

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/3 : FROM alpine AS x
latest: Pulling from library/alpine
88286f41530e: Pull complete
Digest: sha256:1072e499f3f655a032e88542330cf75b02e7bdf673278f701d7ba61629ee3ebe
Status: Downloaded newer image for alpine:latest
 ---> 7328f6f8b418
Step 2/3 : FROM alpine AS y
 ---> 7328f6f8b418
Step 3/3 : COPY --FROM=x . .
Unknown flag: FROM

[rcjsuen]

The other instruction that supports flags is HEALTHCHECK with a CMD We should also validate these flags.

1. --interval=30s
2. --timeout=30s
3. --start-period=0s
4. --retries=3

[rcjsuen]

Docker doesn't seem to validate the misnamed flags if a NONE is found for a HEALTHCHECK.

$ cat Dockerfile
FROM alpine
HEALTHCHECK --timeo=1s --retrie=5 NONE
$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM alpine
 ---> 7328f6f8b418
Step 2/2 : HEALTHCHECK --timeo=1s --retrie=5 NONE
 ---> Running in 24d5ba296300
 ---> 899ec05d3623
Removing intermediate container 24d5ba296300
Successfully built 899ec05d3623