bake: support composable attributes

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

__tests__/.fixtures/bake-03.hcl

@@ -0,0 +1,28 @@
+// Copyright 2024 actions-toolkit authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+target "default" {
+  cache-from = [
+    "type=gha,scope=build",
+    "user/repo:cache",
+  ]
+  cache-to = [
+    "type=gha,scope=build,mode=max",
+    "type=inline"
+  ]
+  output = [
+    "./release-out",
+    "type=registry,ref=user/app"
+  ]
+}