Installing missing package pv (#443)

I tried to use wal-e as mentioned on dockerhub (see https://hub.docker.com/r/mattermost/mattermost-prod-db). To do the base backup I had to do some additional steps (see https://github.com/mattermost/mattermost-docker/issues/346):
- mount a volume into the database docker container (path in the container is /etc/wal-e.d/env) and put all credentials and the s3 prefix to this folder
- exec into the container and create a superuser (Command: CREATE USER postgres SUPERUSER;)

Now it still fails with the message, that the package *pv* is missing

db/Dockerfile

@@ -12,6 +12,7 @@ RUN apk add --no-cache \
       python-dev \
       py-pip \
       py-cryptography \
+      pv \
     && pip --no-cache-dir install 'wal-e<1.0.0' envdir \
     && rm -rf /var/cache/apk/* /tmp/* /var/tmp/*