Swarm mode check fails on non-standard Info responses (#376)

* Swarm mode check fails on non-standard Info responses

* Add unit test

* Remove balena tests, add note to docs

cmd/backup/stop_restart_test.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/api/types/swarm"
+)
+
+type mockInfoClient struct {
+	result types.Info
+	err    error
+}
+
+func (m *mockInfoClient) Info(context.Context) (types.Info, error) {
+	return m.result, m.err
+}
+
+func TestIsSwarm(t *testing.T) {
+	tests := []struct {
+		name        string
+		client      *mockInfoClient
+		expected    bool
+		expectError bool
+	}{
+		{
+			"swarm",
+			&mockInfoClient{
+				result: types.Info{
+					Swarm: swarm.Info{
+						LocalNodeState: swarm.LocalNodeStateActive,
+					},
+				},
+			},
+			true,
+			false,
+		},
+		{
+			"compose",
+			&mockInfoClient{
+				result: types.Info{
+					Swarm: swarm.Info{
+						LocalNodeState: swarm.LocalNodeStateInactive,
+					},
+				},
+			},
+			false,
+			false,
+		},
+		{
+			"balena",
+			&mockInfoClient{
+				result: types.Info{
+					Swarm: swarm.Info{
+						LocalNodeState: "",
+					},
+				},
+			},
+			false,
+			false,
+		},
+		{
+			"error",
+			&mockInfoClient{
+				err: errors.New("the dinosaurs escaped"),
+			},
+			false,
+			true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := isSwarm(test.client)
+			if (err != nil) != test.expectError {
+				t.Errorf("Unexpected error value %v", err)
+			}
+			if test.expected != result {
+				t.Errorf("Expected %v, got %v", test.expected, result)
+			}
+		})
+	}
+}