Rework Dockerfile to reduce image size

[dumbasPL]

Things that changed:

• split the build process into two stages (added a separate build stage)
• build the app in standalone mode. Based on this example
• switched to using --chown=borgwarehouse:borgwarehouse to avoid an unmercenary copy that comes with a manual chown.
• removed the git apt package (I wasn't able to find where it's used so I'm assuming it's not needed)
• removed the apt upgrade command. IMO this should be done by just pulling a new version of the base image and rebuilding.
• added extra files to .dockerignore to help with caching builds

I decided against trying to move away from the Debian base, so that's actually unchanged. Maybe I will make a separate alpine-based image later.

closes #57

[dumbasPL]

Size reduction

• compressed: from 408.6MB to 122.55MB
• uncompressed: from 1.2GB to 393MB

[hugalafutro]

I've built the image with your changes and did an in-place upgrade of my setup and everything works and the image is 1/3rd of what it was.

[Ravinou]

Thank you very much @dumbasPL , I merged your changes and I'm building the new image to push it on dockerhub.

Thank you for testing it with me @hugalafutro 🙏

[dumbasPL]

I'm building the new image to push it on dockerhub.

Automating that with GitHub actions would probably be a good idea 😉