Handle situation where digest is missing in reigstry response header

Signed-off-by: Philip Laine <philip.laine@gmail.com>

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - [#869](https://github.com/spegel-org/spegel/pull/869) Fix request logging for redirects and not found pages.
 - [#872](https://github.com/spegel-org/spegel/pull/872) Allow returning libp2p crypto priv key in linter.
 - [#894](https://github.com/spegel-org/spegel/pull/894) Update Kind to v0.29.0 and Fix Containerd v2 support.
+- [#899](https://github.com/spegel-org/spegel/pull/899) Handle situation where digest is missing in reigstry response header.
 
 ### Security
 

pkg/oci/client.go

@@ -205,6 +205,9 @@ func (c *Client) fetch(ctx context.Context, method string, dist DistributionPath
 			return nil, ocispec.Descriptor{}, err
 		}
 
+		if resp.Header.Get(HeaderDockerDigest) == "" {
+			resp.Header.Set(HeaderDockerDigest, dist.Digest.String())
+		}
 		desc, err := DescriptorFromHeader(resp.Header)
 		if err != nil {
 			return nil, ocispec.Descriptor{}, err