Merge pull request #1270 from tlbdk/ssh2-support

Inherit extended agent so we get modern sign hashes

session/sshforward/sshprovider/agentprovider.go

@@ -178,7 +178,7 @@ type sock struct {
 }
 
 type readOnlyAgent struct {
-	agent.Agent
+	agent.ExtendedAgent
 }
 
 func (a *readOnlyAgent) Add(_ agent.AddedKey) error {
@@ -196,3 +196,7 @@ func (a *readOnlyAgent) RemoveAll() error {
 func (a *readOnlyAgent) Lock(_ []byte) error {
 	return errors.Errorf("locking agent not allowed by buildkit")
 }
+
+func (a *readOnlyAgent) Extension(_ string, _ []byte) ([]byte, error) {
+	return nil, errors.Errorf("extensions not allowed by buildkit")
+}