Feature: PGP Asymmetric Encryption (#456)

* feat: asym encryption

* tests

* docs

* refactor

* logs & errs

* comment

* Update docs/reference/index.md

use correct env var in example

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* Update cmd/backup/encrypt_archive.go

use errwarp for initial error msg

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

* rm orphaned code in encryption functions

* inline readArmoredKeys

* naming -GPG_PUBLIC_KEYS- to GPG_PUBLIC_KEY_RING

* add eror handling for closing func

* use dynamically generated keys for testing

* rm explicit gpg-agent start

* rm unnecessary private_key export

* pass PASSPHRASE correctly to the decryption command

* capture defer errors

* log & err msg

---------

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>

test/gpg-asym/docker-compose.yml

@@ -0,0 +1,25 @@
+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.gpg
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
+      GPG_PUBLIC_KEY_RING_FILE: /keys/public_key.asc
+    volumes:
+      - ${KEY_DIR:-.}/public_key.asc:/keys/public_key.asc
+      - ${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: