clairctl: add support for s3 registries using V4

S3 signature V4 includes the http method on the signature. To create
URLs that clair can use to fetch layers, clairctl needs to make GET
calls instead of HEAD ones

Fixes #1264

Signed-off-by: Hector Huertas <hectorhuertas@gmail.com>

cmd/clairctl/manifest.go

@@ -136,17 +136,19 @@ func Inspect(ctx context.Context, r string) (*claircore.Manifest, error) {
 		if err != nil {
 			return nil, err
 		}
-		req, err := http.NewRequestWithContext(ctx, http.MethodHead, u.String(), nil)
+		req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
 		if err != nil {
 			return nil, err
 		}
+		req.Header.Add("Range", "bytes=0-0")
 		res, err := c.Do(req)
 		if err != nil {
 			return nil, err
 		}
 		res.Body.Close()
 
 		res.Request.Header.Del("User-Agent")
+		res.Request.Header.Del("Range")
 		out.Layers = append(out.Layers, &claircore.Layer{
 			Hash:    ccd,
 			URI:     res.Request.URL.String(),