downloader: Dockerfile fixes to make it work

downloader/Dockerfile

@@ -1,5 +1,6 @@
 FROM alpine:3.7
-RUN apk --update add py2-pip
+# dependencies needed for compiling c extensions
+RUN apk --update add py2-pip gcc python-dev musl-dev
 
 # Install common lib first as it changes less
 COPY common /tmp/common
@@ -9,4 +10,4 @@ RUN pip install /tmp/common && rm -r /tmp/common
 COPY downloader /tmp/downloader
 RUN pip install /tmp/downloader && rm -r /tmp/downloader
 
-ENTRYPOINT python2 -m downloader --base-dir /mnt
+ENTRYPOINT ["python2", "-m", "downloader", "--base-dir", "/mnt"]