Adds ca-certificates.

Fixes issue #3.

Dockerfile

@@ -1,7 +1,15 @@
 FROM alpine:3.5
 
 RUN set -x \
- &&apk add --no-cache duplicity openssh openssl py-crypto py-pip rsync \
+ && apk add --no-cache \
+        ca-certificates \
+        duplicity \
+        openssh \
+        openssl \
+        py-crypto \
+        py-pip \
+        rsync \
+ && update-ca-certificates \
  && pip install pydrive==1.3.1 \
  && apk del --purge py-pip \
  && adduser -D -u 1896 duplicity \