Simplify build process and fix scripts for Flask 1 compat

Debian has still Flask 1 in stable.

Dockerfile

-FROM registry.git.autistici.org/ai3/docker/s6-base:master
+FROM registry.git.autistici.org/ai3/docker/s6-base:master AS base
 
 RUN apt-get -q update && \
     env DEBIAN_FRONTEND=noninteractive apt-get -qy install --no-install-recommends \
-        python3-flask python3-sqlalchemy python3-flask-sqlalchemy \
-        python3-setuptools ca-certificates && \
+        python3-flask python3-sqlalchemy python3-flask-sqlalchemy python3-cheroot \
+        python3-setuptools python3-pip && \
     apt-get clean && \
     rm -fr /var/lib/apt/lists/*
 
-ADD . /tmp/src
-WORKDIR /tmp/src
-RUN python3 setup.py install
+FROM base AS build
 
+ADD . /src
+WORKDIR /src
+RUN python3 setup.py bdist_wheel
+
+FROM base
+
+COPY --from=build /src/dist/*.whl /tmp/wheels/
 COPY conf/ /etc/
 COPY feedbackloop.sh /usr/local/bin/feedbackloop
+
+RUN cd /tmp/wheels && pip3 install *.whl && rm -fr /tmp/wheels
+
+ENV FLASK_APP=feedbackloop.main

feedbackloop.sh

 #!/bin/sh
 
-exec flask --app feedbackloop.main "$@"
+exec flask "$@"