updated dockerfile to install necessary python3.9

[helmut1337]

hey,

wanted to run this in a docker container with the provided Dockerfile but it didn't work due to wrong python-version being installed in the Dockerfile.

Running twitch-archiver in the container gave me:
Traceback (most recent call last): File "/usr/local/bin/twitch-archiver", line 5, in <module> from twitcharchiver.__init__ import main File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/__init__.py", line 40, in <module> from twitcharchiver.channel import Channel File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/channel.py", line 10, in <module> from twitcharchiver.api import Api File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/api.py", line 9, in <module> from twitcharchiver.exceptions import ( File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/exceptions.py", line 10, in <module> from twitcharchiver.utils import get_temp_dir File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/utils.py", line 20, in <module> from twitcharchiver.twitch import Chapters File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/twitch.py", line 82, in <module> class Chapters: File "/usr/local/lib/python3.8/dist-packages/twitcharchiver/twitch.py", line 88, in Chapters def __init__(self, moments: list[dict] = None): TypeError: 'type' object is not subscriptable

Affected Line:
def __init__(self, moments: list[dict] = None):

Description:
https://docs.python.org/3.9/whatsnew/3.9.html#type-hinting-generics-in-standard-collections

Change:
Installing python v3.9 instead of v3.8.10 (ubuntu default)

[Brisppy]

Thanks for the PR, this will be included in the next release.