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Image annotations

Buildkit supports attaching OCI annotations 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
  • The index
    • This adds the annotation into the image index root
    • If the exporter does not produce an image index, the build will fail.
  • The index-descriptor
    • This adds the annotation into the OCI layout's descriptor for the index
    • If the exporter does not produce an image index, the build will fail.

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"