Update Dockerfile to install everything from piwheels on arm/v7

docker/Dockerfile

@@ -1,14 +1,14 @@
 FROM python:3.9-slim-buster as builder
 
 COPY requirements.txt /tmp
-RUN apt update && apt install -y build-essential libffi-dev libpq-dev
+RUN apt update && apt install -y build-essential libpq-dev
 
 RUN \
     if [ `dpkg --print-architecture` = "armhf" ]; then \
     printf "[global]\nextra-index-url=https://www.piwheels.org/simple\n" > /etc/pip.conf ; \
     fi
 RUN pip wheel --wheel-dir /wheels -r /tmp/requirements.txt
-RUN rm -f /etc/pip.conf && pip wheel --wheel-dir /wheels apprise uwsgi
+RUN pip wheel --wheel-dir /wheels apprise uwsgi
 
 FROM python:3.9-slim-buster