Added simple docker file to run release in a container

[luckydonald]

It downloads the zip from the releases, instead of compiling it, but it allows to run it on a docker server.

So could (should) be improved but maybe is a starting point.

Usage something similar to this:

docker build . -t tg-dl:1.0.5-test.5
docker run -it --rm --name=tg_dl -v "/data/telegram-downloader:/data" tg-dl:1.0.5-test.5 
/bin/bash
# now you should be inside the container
java -jar telegram_backup.jar --target '/data/' --help

[fabianonline]

Thanks a lot for this. I'm using docker a lot, so I'm quite surprised I've never thought about providing the necessary files myself... :D