Postgres: Backup and restore scripts.

postgres/Dockerfile

 FROM docker.io/postgres:16.1-alpine3.19
-COPY --chown=root:root healthcheck /usr/local/bin/
+COPY --chown=root:root healthcheck backup restore /usr/local/bin/
 HEALTHCHECK --start-period=3m CMD healthcheck

postgres/README.md

 # postgres
 
 Just the upstream image but with a healthcheck.
+
+## Backups
+
+The image includes a `backup` and `restore` scripts. The `backup` scripts dumps
+all of the databases using `pg_dumpall` and compresses the output using `zstd`
+to stdout. This is meant so that backups are run by an external process and it
+saves the output to a file, for example:
+
+```
+docker exec pg1 backup > /var/backups/pg1/dump.sql.zstd
+```
+
+The `restore` script matches the `backup` script in that the it reads a zstd
+compress SQL dump from stdin. An example restore:
+
+```
+cat dump.sql.zstd | docker exec -i pg2 restore
+```
+
+In fact you're able to migrate data from 1 instance to another like so:
+
+```
+docker exec pg1 backup | docker exec -i pg2 restore
+```

postgres/backup

+#!/usr/bin/env bash
+set -euo pipefail
+
+export PGUSER="${POSTGRES_USER:-postgres}"
+
+pg_dumpall | zstd

postgres/restore

+#!/usr/bin/env bash
+set -euo pipefail
+
+export PGUSER="${POSTGRES_USER:-postgres}"
+
+zstd --decompress - | psql