container: provide usage example

This requires cgo so unfortunately will not run in the playground.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/check/absolute.go

@@ -56,7 +56,7 @@ func NewAbs(pathname string) (*Absolute, error) {
 // MustAbs calls [NewAbs] and panics on error.
 func MustAbs(pathname string) *Absolute {
 	if a, err := NewAbs(pathname); err != nil {
-		panic(err.Error())
+		panic(err)
 	} else {
 		return a
 	}

container/check/absolute_test.go

@@ -84,9 +84,9 @@ func TestNewAbs(t *testing.T) {
 		t.Parallel()
 
 		defer func() {
-			wantPanic := `path "etc" is not absolute`
+			wantPanic := &AbsoluteError{Pathname: "etc"}
 
-			if r := recover(); r != wantPanic {
+			if r := recover(); !reflect.DeepEqual(r, wantPanic) {
 				t.Errorf("MustAbs: panic = %v; want %v", r, wantPanic)
 			}
 		}()