Pin Docker tag (#29)

Author: Denis-Averin

Dockerfile

@@ -2,7 +2,7 @@
 # docker build github.com/aspose-barcode-cloud/aspose-barcode-cloud-python -t barcode-cloud-python:$(git describe --tags)
 # docker run barcode-cloud-python:$(git describe --tags) publish -e "TEST_CONFIGURATION_ACCESS_TOKEN=" -e "TWINE_PASSWORD="
 
-FROM python:2
+FROM python:2-buster
 
 RUN apt-get update \
 	&& apt-get install -y python3-pip \