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/Dockerfile

@@ -1,8 +1,10 @@
 FROM docker:27-dind
 
 RUN apk add \
+  age \
   coreutils \
   curl \
+  expect \
   gpg \
   gpg-agent \
   jq \