docs: built-in build args

Signed-off-by: CrazyMax <[email protected]>

docs/reference/buildx_bake.md

@@ -246,8 +246,6 @@ $ docker buildx bake -f docker-bake.hcl --print db
 Same as [`build --progress`](buildx_build.md#progress). Set type of progress
 output (auto, plain, tty). Use plain to show container output (default "auto").
 
-> You can also use the `BUILDKIT_PROGRESS` environment variable to set its value.
-
 The following example uses `plain` output during the build:
 
 ```console

docs/reference/buildx_build.md

@@ -54,6 +54,19 @@ For documentation on most of these flags, refer to the [`docker build`
 documentation](https://docs.docker.com/engine/reference/commandline/build/). In
 here we’ll document a subset of the new flags.
 
+### Built-in build args
+
+* `BUILDKIT_INLINE_BUILDINFO_ATTRS=<bool>` inline build info attributes in image config or not
+* `BUILDKIT_INLINE_CACHE=<bool>` inline cache metadata to image config or not
+* `BUILDKIT_MULTI_PLATFORM=<bool>` opt into determnistic output regardless of multi-platform output or not
+* `BUILDKIT_PROGRESS=<string>` same as [`--progress` flag](#progress).
+
+```shell
+docker buildx build --build-arg BUILDKIT_PROGRESS=plain .
+```
+
+Other useful build-in args can be found in [dockerfile frontend docs](https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md#built-in-build-args).
+
 ## Examples
 
 ### <a name="builder"></a> Override the configured builder instance (--builder)
@@ -110,9 +123,6 @@ $ docker buildx build --platform=darwin .
 Set type of progress output (auto, plain, tty). Use plain to show container
 output (default "auto").
 
-> You can also use the `BUILDKIT_PROGRESS` environment variable to set
-> its value.
-
 The following example uses `plain` output during the build:
 
 ```console