Ensure libpopt0 is listed in required packages

This is a problem on Linux when using full RPM support otherwise.

See: aboutcode-org/scancode.io#6 (comment)
Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Dockerfile

@@ -1,31 +1,17 @@
 #
 # Copyright (c) nexB Inc. and others. All rights reserved.
-# SPDX-License-Identifier: Apache-2.0 AND CC-BY-4.0
-#
-# Visit https://aboutcode.org and https://github.com/nexB/scancode-toolkit for
-# support and download. ScanCode is a trademark of nexB Inc.
-#
-# The ScanCode software is licensed under the Apache License version 2.0.
-# The ScanCode open data is licensed under CC-BY-4.0.
-#
-# 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.
+# ScanCode is a trademark of nexB Inc.
+# SPDX-License-Identifier: Apache-2.0
+# See http://www.apache.org/licenses/LICENSE-2.0 for the license text.
+# See https://github.com/nexB/scancode-toolkit for support or download.
+# See https://aboutcode.org for more information about nexB OSS projects.
 #
 
 FROM python:3.6-slim-buster 
 
 # Requirements as per https://scancode-toolkit.readthedocs.io/en/latest/getting-started/install.html
 RUN apt-get update \
- && apt-get install -y bzip2 xz-utils zlib1g libxml2-dev libxslt1-dev libgomp1 \
+ && apt-get install -y bzip2 xz-utils zlib1g libxml2-dev libxslt1-dev libgomp1 libpopt0\
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*