feat: allow backups to be encrypted with age (#432)

GPG is known to have usability issues and is generally cumbersome to
use. age [0] is a modern alternative to GPG that is designed by a
cryptographer that has worked and continues to work on Golang's crypto
packages for years.

Allowing age to be used to encrypt backups dramatically simplifies the
backup process.

[0]: https://age-encryption.org/

test/age-passphrase/docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
+    restart: always
+    environment:
+      BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      BACKUP_FILENAME: test.tar.gz
+      BACKUP_LATEST_SYMLINK: test-latest.tar.gz.age
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
+      AGE_PASSPHRASE: "Dance.0Tonight.Go.Typical"
+    volumes:
+      - ${LOCAL_DIR:-./local}:/archive
+      - app_data:/backup/app_data:ro
+      - /var/run/docker.sock:/var/run/docker.sock
+
+  offen:
+    image: offen/offen:latest
+    labels:
+      - docker-volume-backup.stop-during-backup=true
+    volumes:
+      - app_data:/var/opt/offen
+
+volumes:
+  app_data:

test/age-passphrase/run.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "$0")"
+. ../util.sh
+current_test=$(basename "$(pwd)")
+
+export LOCAL_DIR="$(mktemp -d)"
+
+docker compose up -d --quiet-pull
+sleep 5
+
+docker compose exec backup backup
+
+expect_running_containers "2"
+
+TMP_DIR=$(mktemp -d)
+
+# complex usage of expect(1) due to age not have a way to programmatically
+# provide the passphrase
+expect -i <<EOL
+spawn age --decrypt -o "$LOCAL_DIR/decrypted.tar.gz" "$LOCAL_DIR/test.tar.gz.age"
+expect -exact "Enter passphrase: "
+send -- "Dance.0Tonight.Go.Typical\r"
+sleep 1
+EOL
+tar -xf "$LOCAL_DIR/decrypted.tar.gz" -C "$TMP_DIR"
+
+if [ ! -f "$TMP_DIR/backup/app_data/offen.db" ]; then
+  fail "Could not find expected file in untared archive."
+fi
+rm -vf "$LOCAL_DIR/decrypted.tar.gz"
+
+pass "Found relevant files in decrypted and untared local backup."
+
+if [ ! -L "$LOCAL_DIR/test-latest.tar.gz.age" ]; then
+  fail "Could not find local symlink to latest encrypted backup."
+fi