Update Dockerfile handling to keep parser directives at the top

Signed-off-by: CrazyMax <google@crazymax.dev>

testdata/expected/file2.Dockerfile

@@ -0,0 +1,17 @@
+# syntax=docker/dockerfile:1.3
+# Copyright 2018 Google LLC
+#
+# 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.
+
+FROM scratch
+CMD ["echo", "hello world"]

testdata/expected/file3.Dockerfile

@@ -0,0 +1,18 @@
+# escape=`
+# Copyright 2018 Google LLC
+#
+# 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.
+
+FROM microsoft/nanoserver
+COPY testfile.txt c:\
+RUN dir c:\

testdata/initial/file2.Dockerfile

@@ -0,0 +1,3 @@
+# syntax=docker/dockerfile:1.3
+FROM scratch
+CMD ["echo", "hello world"]

testdata/initial/file3.Dockerfile

@@ -0,0 +1,4 @@
+# escape=`
+FROM microsoft/nanoserver
+COPY testfile.txt c:\
+RUN dir c:\