Docker is nice. The main benefit is volume mounts to avoid unknown dependencies on the host file system.

Nix is nicer and lower (runtime) overhead. It has fairly nice declarative config management, but not too sure how declarative data management is.

Nix would probably be more secure (can set up dedicated users for each job, but this is extra overhead for new apps)

Docker is nicer for composing services, but I don't do any of that right now.

Consider using Restic instead of Borg (+ borgmatic).

Why

Support for non-SSH storage - allows cheaper object storage like B2. Using object storage allows provisioning the backup space with Terraform (the only remaining manual bit of the server setup).

Go vs Python - minor point, Python is hard to package and use in some systems.

Cons

Borg is more widely used.

Borgmatic is a nice wrapper. How is restic CLI and does it need similar? If yes, then more investigation is needed. If no, then getting rid of a dependency is a pro.

Borg supports compression and is apparently faster (minor point). Compression is meh relevancy, because server doesn't have too much data. Faster is nice, but not required for a non-interactive backup. However, may limit backup usage in other applications.