Fix alpine 3.13 APK proxy build issue

Alpine 3.13 set APK repositories to https instead of http.
APK does not work with https proxy because of libfetch issue.
Set alpine to 3.12 until issue is resolved.

Signed-off-by: Kennelly, Martin <martin.kennelly@intel.com>

images/Dockerfile

@@ -11,7 +11,7 @@ RUN apk add --update --virtual build-dependencies build-base linux-headers && \
     make build && \
     cd /tmp/ddptool && make
 
-FROM alpine
+FROM alpine:3.12
 RUN apk add hwdata-pci
 COPY --from=builder /usr/src/sriov-network-device-plugin/build/sriovdp /usr/bin/
 COPY --from=builder /tmp/ddptool/ddptool /usr/bin/