Docker container for JDownlaoder2 on aarch64

This is a fork based on jlesage/jdownloader-2, and modified to run on aarch64 machines.

Refer to jlesage/jdownloader-2 for usage and setup, the packages are equivalent.

Tested on Raspberry Pi 4 4GB running Ubuntu 20.04 (64bit)

Changes

• baseimage: use Ubuntu 20.04, for better support on aarch64 packages
• openjdk-8-jre: From Ubuntu focal repository
• 7-Zip-Bindings V16.02 built for Linux-aarch64