unencapsulate: Handle legacy docker MIME type

In the original design of the skopeo proxy, the idea is that
it shields us from the legacy Docker formats, converting everything
to OCI.

That has worked - except it turns out in the case of pulling
a docker-formatted image from `containers-storage`, the proxy
code fails to convert the layer types.

In this case, it's easy to handle here; we only need
to care about the uncompressed version in this case.

lib/src/container/unencapsulate.rs

@@ -45,6 +45,11 @@ use tokio::{
 };
 use tracing::instrument;
 
+/// The legacy MIME type returned by the skopeo/(containers/storage) code
+/// when we have local uncompressed docker-formatted image.
+/// TODO: change the skopeo code to shield us from this correctly
+const DOCKER_TYPE_LAYER_TAR: &str = "application/vnd.docker.image.rootfs.diff.tar";
+
 type Progress = tokio::sync::watch::Sender<u64>;
 
 /// A read wrapper that updates the download progress.
@@ -194,6 +199,7 @@ fn new_async_decompressor<'a>(
             async_compression::tokio::bufread::GzipDecoder::new(src),
         ))),
         oci_image::MediaType::ImageLayer => Ok(Box::new(src)),
+        oci_image::MediaType::Other(t) if t.as_str() == DOCKER_TYPE_LAYER_TAR => Ok(Box::new(src)),
         o => Err(anyhow::anyhow!("Unhandled layer type: {}", o)),
     }
 }