Make sure we use newer version of ubuntu to get updated openssh

ssh/kms.go

@@ -59,7 +59,7 @@ func (kmss *KMSSigner) Sign(rand io.Reader, digest []byte, opts crypto.SignerOpt
 	// Query the API.
 	res, err := kmss.client.AsymmetricSign(kmss.ctx, req)
 	if err != nil {
-		return nil, fmt.Errorf("%v\nhash: %v", err, hashLookup[opts.HashFunc()])
+		return nil, fmt.Errorf("%v\nrequested hash: %v", err, hashLookup[opts.HashFunc()])
 	}
 
 	return res.Signature, nil

testdata/Dockerfile

@@ -1,5 +1,5 @@
 # syntax = docker/dockerfile:experimental
-FROM ubuntu as builder
+FROM ubuntu:19.10 as builder
 
 # Test ssh key injection
 RUN apt-get update -qq && \