docs/build: add note about git subdirectories with BuildKit
@@ -110,6 +110,11 @@ Build Syntax Suffix | Commit Used | Build Context Used
`myrepo.git#mytag:myfolder` | `refs/tags/mytag` | `/myfolder`
`myrepo.git#mybranch:myfolder` | `refs/heads/mybranch` | `/myfolder`
+> **Note**
+>
+> You cannot specify the build-context directory (`myfolder` in the examples above)
+> when using BuildKit as builder (`DOCKER_BUILDKIT=1`). Support for this feature
+> is tracked in [buildkit#1684](https://github.com/moby/buildkit/issues/1684).
### Tarball contexts
