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/lint/container-entrypoint.sh

@@ -1,4 +1,9 @@
 #!/usr/bin/env bash
+#
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
 export LC_ALL=C
 
 # Fixes permission issues when there is a container UID/GID mismatch with the owner