Add database to filename, don't install recommends

Cron comes with just a metric ton of recommended dependencies including
mariadb-common which is just a bunch of unneccessary weight. Just
install what's necessary for cron.

postgres-docker/Dockerfile

@@ -5,7 +5,7 @@ RUN mkdir /backups
 COPY ./backup.sh /backups
 COPY ./weed.sh /backups
 COPY ./cronfile /etc/cron.d/cronfile
-RUN apt-get update && apt-get -y install cron
+RUN apt-get update && apt-get -y --no-install-recommends install cron
 RUN chmod 0644 /etc/cron.d/cronfile
 RUN crontab /etc/cron.d/cronfile
 RUN touch /var/log/cron.log

postgres-docker/backup.sh

@@ -3,5 +3,5 @@ if [ -z "$POSTGRES_DB" ]; then
     echo "Database not specified, defaulting to bookwyrm"
 fi
 BACKUP_DB=${POSTGRES_DB:-bookwyrm}
-filename=backup__$(date +%F)
+filename=backup_${BACKUP_DB}_$(date +%F)
 pg_dump -U $BACKUP_DB > /backups/$filename.sql