Matrix: Add backups.

- Mark the Synapse data volume for snapshot by `dvb` (it just a bunch of
  files anyway).
- Do weekly backups of the database.

Compose/matrix/crond/.dockerignore

+*
+!backup
+!crontab

Compose/matrix/crond/Dockerfile

+
+# hadolint ignore=DL3006
+FROM registry.shore.co.il/cron as supersonic
+# hadolint ignore=DL3002
+USER root
+# hadolint ignore=DL3018
+RUN apk add --update --no-cache \
+    docker-cli \
+    findutils \
+    ;
+COPY --chown=root:root backup /usr/local/bin/
+COPY --chown=root:root restore /usr/local/bin/
+VOLUME /var/backups

Compose/matrix/crond/README.md

+# crond
+
+Runs background jobs for the Matrix server. The jobs are:
+
+- Postgres DB backup.

Compose/matrix/crond/backup

+#!/bin/sh
+set -eu
+
+now="$(date --utc -Iseconds)"
+dest="/var/backups/${now}.sql.zstd"
+mkdir --parents "$(dirname "$dest")"
+docker exec matrix-postgres-1 backup > "$dest"

Compose/matrix/crond/crontab

+@weekly backup || wget --spider https://notify.shore.co.il/send?message=Matrix%20DB%20backup%20failed.
+@daily find /var/backups -atime +18 -delete

Compose/matrix/docker-compose.yaml

@@ -2,6 +2,16 @@
 version: '3.5'
 
 services:
+  cron:
+    build:
+      context: crond/
+    depends_on:
+      - postgres
+    restart: always
+    volumes:
+      - /run/docker.sock:/run/docker.sock
+      - /var/backups/matrix:/var/backups
+
   postgres:
     environment:
       POSTGRES_DB: &postgres_db synapse
@@ -39,6 +49,8 @@ services:
 
 volumes:
   data:
+    labels:
+      snapshot: 'true'
   postgres:
   _run_slapd:
     external: true