Improve host fallback behaviour in docker remote

This commit improves the fallback behaviour when resolving and
fetching images with multiple hosts. If an error is encountered
when resolving and fetching images, and more than one host is being
used, we will try the same operation on the next host. The error
from the first host is preserved so that if all hosts fail, we can
display the error from the first host.

fixes #3850

Signed-off-by: Alex Price <aprice@atlassian.com>

remotes/docker/resolver.go

@@ -286,7 +286,11 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 				if errors.Cause(err) == ErrInvalidAuthorization {
 					err = errors.Wrapf(err, "pull access denied, repository does not exist or may require authorization")
 				}
-				return "", ocispec.Descriptor{}, err
+				// Store the error for referencing later
+				if lastErr == nil {
+					lastErr = err
+				}
+				continue // try another host
 			}
 			resp.Body.Close() // don't care about body contents.