fix: add prefer-binary flag to pip install (#74)

To stop pip from trying to build from source

Dockerfile

@@ -7,7 +7,7 @@ COPY requirements.txt .
 # If platform is arm then we add the piwheels index for prebuilt arm wheels
 RUN if [ $(uname -m | cut -c 1-3) = "arm" ]; then \
     echo -e "[global]\nextra-index-url=https://www.piwheels.org/simple" > /usr/local/pip.conf; fi && \
-    pip --no-cache-dir install -r requirements.txt
+    pip --no-cache-dir install -r requirements.txt --prefer-binary
 
 COPY . .