chore: Add explicit digest requirement to docker pusher

The `push` function below assumes that digest and mediatypes are
populated and set. If they aren't, then the requests made are malformed,
attempting to invoke `HEAD /blobs/` (instead of `HEAD /blobs/<digest>`).
Additionally, if we *were* to move past this point, we'd then populate
an empty digest in the query parameter, and even provide invalid HTTP
mediatypes.

However, the `WithDescriptor` `WriterOpt` specifically notes that "Write
does not require any field of desc to be set". It's very easy for the
caller to read this as an optional field, to skip it, and then get
confusing HTTP errors from inside the `push` function.

We can avoid this by explicitly validating that the descriptor is valid
and provide early feedback.

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

core/remotes/docker/pusher.go

@@ -60,6 +60,12 @@ func (p dockerPusher) Writer(ctx context.Context, opts ...content.WriterOpt) (co
 	if wOpts.Ref == "" {
 		return nil, fmt.Errorf("ref must not be empty: %w", errdefs.ErrInvalidArgument)
 	}
+	if wOpts.Desc.Digest == "" {
+		return nil, fmt.Errorf("descriptor digest must not be empty: %w", errdefs.ErrInvalidArgument)
+	}
+	if wOpts.Desc.MediaType == "" {
+		return nil, fmt.Errorf("descriptor media type must not be empty: %w", errdefs.ErrInvalidArgument)
+	}
 	return p.push(ctx, wOpts.Desc, wOpts.Ref, true)
 }