Cannot "FROM" a previous stage whose name contains uppercase letters

When performing multi-stage builds with BuildKit, I find it impossible to "FROM" a previous stage whose name contains uppercase letters.

A simple Dockerfile as following:

FROM alpine:3 AS TEST

FROM TEST

gives the error repository name must be lowercase:

Dockerfile:3
--------------------
   1 |     FROM alpine:3 AS TEST
   2 |
   3 | >>> FROM TEST
   4 |
--------------------
error: failed to solve: rpc error: code = Unknown desc = failed to solve with frontend dockerfile.v0: failed to create LLB definition: failed to parse stage name "TEST": invalid reference format: repository name must be lowercase

However, Docker just accepts it when using docker build:

Sending build context to Docker daemon  160.3kB
Step 1/2 : FROM alpine:3 AS TEST
 ---> 7731472c3f2a
Step 2/2 : FROM TEST
 ---> 7731472c3f2a
Successfully built 7731472c3f2a

I hope BuildKit can recognize the name is of a previous stage, not a repository.

Start by reproducing the issue with the Dockerfile shown and the BuildKit Dockerfile frontend, then trace how stage names are parsed when resolving the second FROM. Done means an uppercase previous-stage name resolves like the lowercase case and the behavior is covered by a regression test.