Port Dockerfile to Ubuntu 20.04

[DangerOnTheRanger]

This PR will merge a Dockerfile that uses Ubuntu 20.04 LTS and not Alpine Linux.

[DangerOnTheRanger]

@samip5 This is the solution I was talking about over here. I didn't include libffi-dev, but I started up and tested Maniwani in devmode just fine, both reading and posting threads. I can't fully test with the docker-compose stack due to an outstanding issue with captchouli (that I suspect is almost certainly due to gelbooru changing up something on their end), but I did test everything but posting.

According to at least one of the issues I linked, installing libffi-dev shouldn't be necessary since Ubuntu uses glibc and not musl, but if you or anyone else encounter problems, it'll be easy to add back installation of libffi-dev.

[samip5]

@samip5 This is the solution I was talking about over here. I didn't include libffi-dev, but I started up and tested Maniwani in devmode just fine, both reading and posting threads. I can't fully test with the docker-compose stack due to an outstanding issue with captchouli (that I suspect is almost certainly due to gelbooru changing up something on their end), but I did test everything but posting.

According to at least one of the issues I linked, installing libffi-dev shouldn't be necessary since Ubuntu uses glibc and not musl, but if you or anyone else encounter problems, it'll be easy to add back installation of libffi-dev.

Actually I did get it building on dev-mode with alpine but it did require that libffi-dev package to be installed.