Build failed with buildkit experimental features

**Is this a BUG REPORT or FEATURE REQUEST?**:
FEATURE REQUEST

**What happened?**:
`az acr task run` failed since the syntax of `FROM` with `--platform` in the Dockerfile (see [example](https://github.com/docker/buildx/blob/master/Dockerfile)) is not supported.

**What did you expect to happen?**:
Build normally.

**How do you reproduce it (as minimally and precisely as possible)?**:

**Environment**:

**Anything else we need to know?**: