Merge pull request #21744 from dvdksn/buildkit_progress_quiet

build: add BUILDKIT_PROGRESS=quiet

Author: dvdksn

content/manuals/build/building/variables.md

@@ -375,6 +375,7 @@ Sets the type of the BuildKit progress output. Valid values are:
 - `auto` (default)
 - `plain`
 - `tty`
+- `quiet`
 - `rawjson`
 
 Usage: