Upgrades Alpine (and duplicity to 0.7.13.1 and above).

Fixes issue #7.

Dockerfile

@@ -1,4 +1,6 @@
-FROM alpine:3.6
+FROM alpine:3.7
+
+ENV HOME=/home/duplicity
 
 RUN set -x \
  && apk add --no-cache \
@@ -12,14 +14,15 @@ RUN set -x \
         py-setuptools \
         rsync \
  && update-ca-certificates \
- && pip install pydrive==1.3.1 \
+ && pip install \
+      pydrive==1.3.1 \
+      fasteners==0.14.1 \
  && apk del --purge py-pip \
  && adduser -D -u 1896 duplicity \
  && mkdir -p /home/duplicity/.cache/duplicity \
  && mkdir -p /home/duplicity/.gnupg \
- && chmod -R go+rwx /home/duplicity/
-
-ENV HOME=/home/duplicity
+ && chmod -R go+rwx /home/duplicity/ \
+ && su - duplicity -c 'duplicity --version'
 
 VOLUME ["/home/duplicity/.cache/duplicity", "/home/duplicity/.gnupg"]