Dockerize

[eritbh]

Prepares the bot to run in Docker. Converts all our configuration to be handled through environment variables rather than a config file, since Docker plays nicer with that. Adds a Dockerfile for an image which hosts the backend on port 8080, and a workflow to automatically build the image and publish it to the Github container registry so we can pull from there for deployments. See also r-anime/misato-frontend#14.

[eritbh]

Suggested nginx config to adapt to this and the frontend both being in their own containers (this still needs to be tested):

server {
	server_name discord.r-anime.moe;
	listen 80;

	location = /.well-known/security.txt {
		# ...
	}
	location ~ ^(api|auth)/ {
		# use whatever port the backend container's 8080 is mapped to
		proxy_pass http://localhost:8081;
	}
	location / {
		# use whatever port the frontend container's 8080 is mapped to
		proxy_pass http://localhost:8082;
	}
}