Switch to using alpine based image for repo2docker

This cuts image size from under 300MB to under 100MB. Plus
we will never actually need this to have additional libraries
installed, since it's talking to the docker socket directly
to build new images. So switching to alpine here is worth it.

Dockerfile

@@ -1,12 +1,5 @@
-FROM ubuntu:17.04
+FROM python:3.6.3-alpine3.6
 
-RUN apt-get update && \
-    apt-get install --yes --no-install-recommends \
-            python3 \
-            python3-pip \
-            python3-setuptools \
-            git && \
-    apt-get clean && apt-get purge
 
 RUN mkdir /tmp/src
 ADD . /tmp/src