Update arm32v7.Dockerfile

Author: jkewley

modules/SenseHatDisplay/arm32v7.Dockerfile

@@ -1,5 +1,6 @@
-FROM balenalib/raspberrypi3
-# The balena base image for building apps on Raspberry Pi 3.
+FROM balenalib/raspberrypi3:stretch
+# The balena base image for building apps on Raspberry Pi 3. 
+# Raspbian Stretch required for piwheels support. https://downloads.raspberrypi.org/raspbian/images/raspbian-2019-04-09/
 
 RUN echo "BUILD MODULE: SenseHatDisplay"
 
@@ -39,4 +40,4 @@ RUN [ "cross-build-end" ]
 ADD /app/ .
 ADD /build/ .
 
-ENTRYPOINT ["python3","-u", "./main.py"]
+ENTRYPOINT ["python3","-u", "./main.py"]