[docker] Switch to Ubuntu 20.04

It successfully passes tests on GitHub CI

Dockerfile

@@ -1,19 +1,13 @@
-FROM alpine:3.12
-# fetching cheat sheets
-## installing dependencies
-RUN apk add --update --no-cache git py3-six py3-pygments py3-yaml py3-gevent \
-      libstdc++ py3-colorama py3-requests py3-icu py3-redis
-## building missing python packages
-RUN apk add --no-cache --virtual build-deps py3-pip g++ python3-dev \
-    && pip3 install --no-cache-dir rapidfuzz colored polyglot pycld2 \
-    && apk del build-deps
-## copying
+FROM ubuntu:20.04
+
+### copying app sources
 WORKDIR /app
 COPY . /app
-RUN mkdir -p /root/.cheat.sh/log/ \
-    && python3 lib/fetch.py fetch-all
 
-# installing server dependencies
-RUN apk add --update --no-cache py3-jinja2 py3-flask bash gawk
+RUN pip install --upgrade -r requirements.txt
+
+## fetching cheat sheets
+RUN python3 lib/fetch.py fetch-all
+
 ENTRYPOINT ["python3", "-u", "bin/srv.py"]
 CMD [""]