docs/build: add note about git subdirectories with BuildKit

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit b4db7e38bcf7a16e2d195fb11140290578a81d6a)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 9a65c1c861c1199039b8a3c0cb6dc73f3acfe9b3
Component: cli

Author: thaJeztah

components/cli/docs/reference/commandline/build.md

@@ -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).
 
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