Merge pull request #3961 from tonistiigi/buildkit-default-policy

policy: verify BuildKit builder images

docs/reference/buildx.md

@@ -45,3 +45,11 @@ Extended build capabilities with BuildKit
 ### <a name="builder"></a> Override the configured builder instance (--builder)
 
 You can also use the `BUILDX_BUILDER` environment variable.
+
+### Enable the default policy
+
+Set `BUILDX_DEFAULT_POLICY=1` to enable Buildx's built-in source policy. The
+policy verifies signed tags for images managed by Docker, including BuildKit
+builder images and Dockerfile frontends. Untagged digest references and images
+outside the managed repositories are allowed unchanged. Tagged references
+that also contain a digest still have their release identity verified.

docs/reference/buildx_create.md

@@ -173,6 +173,18 @@ documentation for the specific driver:
 * [`kubernetes` driver](https://docs.docker.com/build/builders/drivers/kubernetes/)
 * [`remote` driver](https://docs.docker.com/build/builders/drivers/remote/)
 
+With `BUILDX_DEFAULT_POLICY=1`, the `docker-container` driver verifies signed
+`moby/buildkit` builder image tags before creating the builder. To explicitly
+bypass this verification, set `allow-untrusted-image=true`. For example:
+
+```console
+$ BUILDX_DEFAULT_POLICY=1 docker buildx create --driver docker-container \
+    --driver-opt allow-untrusted-image=true
+```
+
+Only use this option for an image that you trust. It disables builder image
+verification for the new builder node.
+
 ### <a name="leave"></a> Remove a node from a builder (--leave)
 
 The `--leave` flag changes the action of the command to remove a node from a