fix: retry docker volume remove

cli/app/remove.go

@@ -148,6 +148,9 @@ flag.
 	},
 }
 
+// retryFunc retries the given function for the given retries. After the nth
+// retry it waits (n + 1)^2 seconds before the next retry (starting with n=0).
+// It returns an error if the function still failed after the last retry.
 func retryFunc(retries int, fn func() error) error {
 	for i := 0; i < retries; i++ {
 		err := fn()

cli/app/remove_test.go

@@ -0,0 +1,26 @@
+package app
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestRetryFunc(t *testing.T) {
+	err := retryFunc(1, func() error { return nil })
+	if err != nil {
+		t.Errorf("should not return an error: %s", err)
+	}
+
+	i := 0
+	fn := func() error {
+		i++
+		return fmt.Errorf("oh no, something went wrong!")
+	}
+	err = retryFunc(2, fn)
+	if err == nil {
+		t.Error("should return an error")
+	}
+	if i != 2 {
+		t.Errorf("The function should have been called 1 times, got %d", i)
+	}
+}