Add docker tags for major versions

[Quadrubo]

Description

As of the point of writing this, v1.8.13 is the latest Borgmatic version. As I'm manually updating my images and not using something like "latest", I have to change the image tag every new release. It would be nice if I wouldn't have to do that for minor releases.

Currently these tags are provided:

• 1.8.13 - Version 1.8.13
• 1.8.12 - Version 1.8.12
• 1.8 - Version 1.8.13
• latest - Version 1.8.13

Desired Behavior

I would propose a tag system like this:

• 1.8.13 - Version 1.8.13
• 1.8.12 - Version 1.8.12
• 1.8 - Version 1.8.13
• 1 - Version 1.8.13
• latest - Version 1.8.13

This would allow pinning to specific version and allows the user to choose which semver types they are okay with auto updating. I for example would have pinned to 1 and would have automatically gotten the updates up to 1.8.13. I will also eventually get 1.9.x without getting 2.x which right now is impossible.