build: clarify default image store limitation

Signed-off-by: David Karlsson <[email protected]>

content/build/attestations/_index.md

@@ -47,6 +47,19 @@ You can opt in to add either the SBOM or provenance attestation type, or both.
 $ docker buildx build --sbom=true --provenance=true .
 ```
 
+> **Note**
+>
+> The default image store doesn't support attestations. If you're using the
+> default image store and you build an image using the default `docker` driver,
+> or using a different driver with the `--load` flag, the attestations are
+> lost.
+>
+> To make sure the attestations are preserved, you can:
+>
+> - Use a `docker-container` driver with the `--push` flag to push the image to
+>   a registry directly.
+> - Enable the [containerd image store](../../desktop/containerd/_index.md).
+
 > **Note**
 >
 > Provenance attestations are enabled by default, with the `mode=min` option.