Allow passing multi-line files

Signed-off-by: Yuanhao Ji <jiyuanhao@apache.org>

test/multi-files/docker-bake.hcl

@@ -0,0 +1,15 @@
+group "default" {
+  targets = ["t3"]
+}
+
+target "t3" {
+  name = "${item.tag}"
+  matrix = {
+    item = t3
+  }
+  args = {
+    VERSION = "${item.version}"
+    DUMMY_ARG = "${item.arg}"
+  }
+  tags = ["${item.tag}"]
+}