Update Dockerfile to Alpine 3.6 and libressl

docker/Dockerfile

@@ -1,12 +1,11 @@
 # Dockerfile for Azure/blobxfer
 
-FROM gliderlabs/alpine:3.4
+FROM alpine:3.6
 MAINTAINER Fred Park <https://github.com/Azure/blobxfer>
 
-RUN apk add --update --no-cache musl build-base python3 python3-dev openssl-dev libffi-dev ca-certificates \
-    && pip3 install --no-cache-dir --upgrade pip \
+RUN apk add --update --no-cache musl build-base python3 python3-dev libressl-dev libffi-dev ca-certificates \
     && pip3 install --no-cache-dir --upgrade blobxfer \
-    && apk del --purge build-base python3-dev openssl-dev libffi-dev \
+    && apk del --purge build-base python3-dev libressl-dev libffi-dev \
     && rm /var/cache/apk/*
 
 ENTRYPOINT ["blobxfer"]