Mysql 8 support, post-backup scripts, ghost 4.x API bug fixes

[unverbraucht]

Hi @bennetimo ,

I made some changes to ghost-backup so it works for my flow. Sorry for the messy PR that includes multiple changes.

I haven't actually tried if the API backup works since I use SQL backups only.

Here's the list of changes: Bump upstream docker image to debian bullseye so we get mysql 8 support in mariadb client. Also allows armv7 and arm64 builds. Shrink image size by cleaning dpkg caches after apt-get. Make db compression optional. Add possibility for post-backup user scripts. Allow manual backup filename override. bug fix: Don't fail if Ghost API is not configured or available.