doc: Add doc comment to ci/test_imagefile

(Similar to the doc comment in ci/lint_imagefile)

Also, rename docker-entrypoint.sh to container-entrypoint.sh

Also, add copyright header to touched files.

ci/test_imagefile

@@ -1,3 +1,9 @@
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+# See ci/README.md for usage.
+
 ARG CI_IMAGE_NAME_TAG
 FROM ${CI_IMAGE_NAME_TAG}