docs: add dedicated annotations guide with examples

The annotations syntax described in the README is helpful as a
reference, but not useful to learn how annotations work. This new piece
of documentation should help fill that role, to help introduce users to
how to use the new feature.

Signed-off-by: Justin Chadwell <me@jedevc.com>

README.md

@@ -257,6 +257,7 @@ Keys supported by image output:
   * Using the extended syntaxes, `annotation-<type>.<key>=<value>`, `annotation[<platform>].<key>=<value>` and both combined with `annotation-<type>[<platform>].<key>=<value>`, allows configuring exactly where to attach the annotation.
   * `<type>` specifies what object to attach to, and can be any of `manifest` (the default), `manifest-descriptor`, `index` and `index-descriptor`
   * `<platform>` specifies which objects to attach to (by default, all), and is the same key passed into the `platform` opt, see [`docs/multi-platform.md`](docs/multi-platform.md).
+  * See [`docs/annotations.md`](docs/annotations.md) for more details.
 
 If credentials are required, `buildctl` will attempt to read Docker configuration file `$DOCKER_CONFIG/config.json`.
 `$DOCKER_CONFIG` defaults to `~/.docker`.

docs/annotations.md

@@ -0,0 +1,59 @@
+# Image annotations
+
+Buildkit supports attaching [OCI annotations](https://github.com/opencontainers/image-spec/blob/main/annotations.md)
+to its built image manifests and indexes. These annotations can be used to
+attach additional metadata to a built image, which may not be appropriate to
+store in the image content itself.
+
+Annotations are similar to, but not a replacement for image labels. Annotations
+can be attached at almost every level of the resulting image output, while
+labels can be only included in the image configuration object. Additionally,
+unless overridden, image labels are inherited by other images that use the
+image as a base.
+
+Annotations support multiple pre-defined annotation keys which you can use, or
+you can also create your own.
+
+To build an image with annotations, you can use the `image` or `oci` (and
+related) exporter types, along with the `annotation.*` option.
+
+For example, to attach a human-readable title to your image, you can use the
+following buildctl invocation:
+
+    buildctl build ... \
+        --opt platform=amd64,arm64 \
+        --output "type=image,name=target,annotation.org.opencontainers.image.title=Target"
+        
+This annotation will be added to each built image manifest, so each platform
+you built for (in the above, `amd64` and `arm64`) will get a copy of the annotation.
+
+You want to allow different annotations for different platforms, e.g. maybe you
+want to provide a different documentation URL per manifest. You can do this
+with platform specific annotations, using the `annotation[<platform>].*` syntax
+like so:
+
+    buildctl build ... \
+        --opt platform=amd64,arm64 \
+        --output "type=image,name=target,annotation[linux/amd64].org.opencontainers.image.url=https://example.com/amd64,annotation[linux/arm64].org.opencontainers.image.url=https://example.com/arm64"
+        
+Buildkit also allows you to finely control the exact destination where the
+annotation will be written to using the syntax `annotation-<type>.*`. You can
+write to the following `<type>`s:
+
+- The `manifest` (the default, as above)
+- The `manifest-descriptor`
+    - This adds the annotation into the image index's descriptor for the manifest
+    - (discarded if the output does not contain an image index)
+- The `index`
+    - This adds the annotation into the image index root
+    - (discarded if the output does not contain an image index)
+- The `index-descriptor`
+    - This adds the annotation into the OCI layout's descriptor for the index
+    - (discarded if the output does not contain an OCI layout)
+
+For example, if you want to add the annotation at the image index level, so
+that the annotation is shared between all architectures, you can instead:
+
+    buildctl build ... \
+        --opt platform=amd64,arm64 \
+        --output "type=image,name=target,annotation-index.org.opencontainers.image.title=Target Image"