fix: format MountType with %v in bind mount diagnostic

docker_mounts.go

@@ -173,7 +173,7 @@ func mapToDockerMounts(containerMounts ContainerMounts) []mount.Mount {
 		case ImageMounter:
 			containerMount.ImageOptions = typedMounter.ImageOptions()
 		case BindMounter:
-			log.Printf("Mount type %s is not supported by Testcontainers for Go", m.Source.Type())
+			log.Printf("Mount type %v is not supported by Testcontainers for Go", m.Source.Type())
 		default:
 			// The provided source type has no custom options
 		}

docker_mounts_test.go

@@ -0,0 +1,45 @@
+package testcontainers
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/testcontainers/testcontainers-go/log"
+)
+
+type captureLogger struct {
+	b *strings.Builder
+}
+
+func (c captureLogger) Printf(format string, v ...any) {
+	fmt.Fprintf(c.b, format, v...)
+}
+
+func TestMapToDockerMountsBindDiagnosticFormat(t *testing.T) {
+	// serial: mutates package log.Default()
+
+	// MountType is a defined uint with no String method; the string verb yields a fmt error marker.
+	badVerb := "%" + "s"
+	require.Contains(t, fmt.Sprintf(badVerb, MountTypeBind), "%!s(")
+	require.Equal(t, "0", fmt.Sprintf("%v", MountTypeBind))
+
+	var buf strings.Builder
+	prev := log.Default()
+	log.SetDefault(captureLogger{b: &buf})
+	t.Cleanup(func() { log.SetDefault(prev) })
+
+	mounts := ContainerMounts{
+		{
+			Source: DockerBindMountSource{HostPath: "/host/path"},
+			Target: "/container/path",
+		},
+	}
+	_ = mounts.PrepareMounts()
+
+	msg := buf.String()
+	require.NotContains(t, msg, "%!s(")
+	require.Contains(t, msg, "Mount type 0 is not supported by Testcontainers for Go")
+}