Pin fpm to 1.15.1 (#2644)

* Pin fpm to 1.15.1

* Explain why

Author: sfc-gh-daniszewski

scripts/packaging/Dockerfile

@@ -30,7 +30,8 @@ RUN apt install -y --fix-missing \
 RUN DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt install -y tk-dev
 
 RUN gem install dotenv -v 2.8.1
-RUN gem install fpm
+# Pin fpm to 1.15.1 for Ruby 2.5 compatibility (Newer versions require Ruby 2.6+ that is not available in Ubuntu 18.04)
+RUN gem install fpm -v 1.15.1
 
 # Copy and run Python build script
 COPY build_python.sh /tmp/build_python.sh