refactor: move act under internal

The runner is an application, not a library. `act/model` and
`act/exprparser` were the last packages anything outside this repository
consumed and they now live in actionslib, so nothing needs the rest of
`act` to be importable, and keeping it importable invites the coupling
that was just removed.

Import paths only, the files are unchanged.

Assisted-by: Codet:GPT-5.1-Codex

internal/act/container/docker_platform_test.go

@@ -0,0 +1,63 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestParsePlatform(t *testing.T) {
+	t.Run("empty input returns nil platform without error", func(t *testing.T) {
+		got, err := parsePlatform("")
+		require.NoError(t, err)
+		assert.Nil(t, got)
+	})
+
+	t.Run("os/arch", func(t *testing.T) {
+		got, err := parsePlatform("linux/amd64")
+		require.NoError(t, err)
+		require.NotNil(t, got)
+		assert.Equal(t, "linux", got.OS)
+		assert.Equal(t, "amd64", got.Architecture)
+		assert.Empty(t, got.Variant)
+	})
+
+	t.Run("os/arch/variant", func(t *testing.T) {
+		got, err := parsePlatform("linux/arm/v7")
+		require.NoError(t, err)
+		require.NotNil(t, got)
+		assert.Equal(t, "linux", got.OS)
+		assert.Equal(t, "arm", got.Architecture)
+		assert.Equal(t, "v7", got.Variant)
+	})
+
+	t.Run("input is lowercased", func(t *testing.T) {
+		got, err := parsePlatform("Linux/AMD64/V8")
+		require.NoError(t, err)
+		require.NotNil(t, got)
+		assert.Equal(t, "linux", got.OS)
+		assert.Equal(t, "amd64", got.Architecture)
+		assert.Equal(t, "v8", got.Variant)
+	})
+
+	for _, bad := range []string{
+		"amd64",
+		"linux",
+		"linux/",
+		"/amd64",
+		"/",
+		"//",
+		"linux/arm/",
+		"linux/arm/v7/extra",
+	} {
+		t.Run("rejects "+bad, func(t *testing.T) {
+			got, err := parsePlatform(bad)
+			require.Error(t, err)
+			assert.Nil(t, got)
+		})
+	}
+}