Make agent usable for external backends (#3270)

cli/exec/exec.go

@@ -213,7 +213,11 @@ func execWithAxis(c *cli.Context, file, repoPath string, axis matrix.Axis) error
 	}
 
 	backendCtx := context.WithValue(c.Context, backendTypes.CliContext, c)
-	backend.Init()
+	backend.Init([]backendTypes.Backend{
+		docker.New(),
+		local.New(),
+		kubernetes.New(),
+	})
 
 	backendEngine, err := backend.FindBackend(backendCtx, c.String("backend-engine"))
 	if err != nil {