Publish diagnostic that warns about a duplicate build stage name

[rcjsuen]

If a build stage name is reused in a Dockerfile, the Docker engine will create an error about this. We should publish a diagnostic for this same error.

FROM alpine AS setup
EXPOSE 8080
FROM alpine AS setup
EXPOSE 8081

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/4 : FROM alpine AS setup
 ---> 7328f6f8b418
Step 2/4 : EXPOSE 8080
 ---> Running in ff3ae57dd33a
 ---> f2a118dba9f0
Removing intermediate container ff3ae57dd33a
Step 3/4 : FROM alpine AS setup
duplicate name setup

[rcjsuen]

Build stage names appear to be case-insensitive.

FROM alpine as setUP
FROM alpine as setup

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/2 : FROM alpine as setUP
latest: Pulling from library/alpine
88286f41530e: Pull complete
Digest: sha256:1072e499f3f655a032e88542330cf75b02e7bdf673278f701d7ba61629ee3ebe
Status: Downloaded newer image for alpine:latest
 ---> 7328f6f8b418
Step 2/2 : FROM alpine as setup
duplicate name setup

Also note that the name in the error is written in lowercase even though the first build stage that was declared had uppercase characters in it.