Base image drinternet/rsync as first party code

[Burnett01]

Discussed in #62

^{Originally posted by Burnett01 June 27, 2024}

This action has been around and maintained for many years with 3.500+ projects depending on it.
As per demand, it is time to streamline/merge the base image code into this action code base.

In the next major version, the base image dependency (drinternet/rsync) will be included as a first party code into this project. This allows for easier code review and Q/A conducted by action users.
This also means that the action will have zero dependencies (other than alpine itself of course).
Both licenses are compatible (MIT) which should ease the migration/merge path.

Since the maintainer of the base image has always been a collaborator of this action,
nothing will change except that regular alpine version updates are done here as well.

[Burnett01]

Feel free to join the discussion in #62 and to keep track on the progress.