[skip ci] license: add license to Dockerfiles

Dockerfile

 # -*- coding: utf-8 -*-
+#
+# This file is part of CERN Search.
+# Copyright (C) 2018-2019 CERN.
+#
+# CERN Search is free software; you can redistribute it and/or modify it
+# under the terms of the MIT License; see LICENSE file for more details.
 
 # Use CentOS7:
 FROM gitlab-registry.cern.ch/webservices/cern-search/cern-search-rest-api/cern-search-rest-api-base:d1f96a1c3600a9a00e76433b593d0edc0a49e532

Dockerfile-base

 # -*- coding: utf-8 -*-
+#
+# This file is part of CERN Search.
+# Copyright (C) 2018-2019 CERN.
+#
+# CERN Search is free software; you can redistribute it and/or modify it
+# under the terms of the MIT License; see LICENSE file for more details.
 
 # Use CentOS7:
 FROM inveniosoftware/centos7-python:3.6