refactor: simplify the final image

Dockerfile

@@ -9,7 +9,7 @@ RUN pip3 wheel --wheel-dir=/wheel . \
     errbot errbot[irc] errbot[XMPP] errbot[telegram] && \
     cp /tmp/slackv3/requirements.txt /wheel/slackv3-requirements.txt
 
-FROM python:3.9-slim
+FROM python:3.9-slim as base
 COPY --from=build /wheel /wheel
 RUN apt update && \
     apt install -y git && \
@@ -18,12 +18,13 @@ RUN apt update && \
     -r /wheel/slackv3-requirements.txt \
     errbot errbot[irc] errbot[XMPP] errbot[telegram] && \
     rm -rf /wheel /var/lib/apt/lists/*
-
 RUN useradd -m errbot
+
+FROM base
+EXPOSE 3141 3142
+VOLUME /home/errbot
 WORKDIR /home/errbot
 USER errbot
 RUN errbot --init
 RUN git clone https://github.com/errbotio/err-backend-slackv3 backend-plugins/slackv3
-EXPOSE 3141 3142
-VOLUME /home/errbot
 ENTRYPOINT [ "/usr/local/bin/errbot" ]