fix: use Debian repo for Raspbian Trixie

[nanookclaw]

- What I did

Closes #542. For Raspbian/Raspberry Pi OS Trixie, install.sh now uses Docker's Debian Trixie APT repository and GPG key path instead of the non-existent linux/raspbian trixie repository. Existing Raspbian releases with published Docker Raspbian repositories stay unchanged.

- How I did it

The script keeps lsb_dist=raspbian for detection and release handling, but uses a separate repository distro variable when building the APT repo and key URL. That variable is mapped to debian only for raspbian + trixie.

- How to verify it

I ran:

git diff --check
sh -n install.sh
/tmp/github-contribute-docker-docker-install-542/verify-raspbian-repo.sh ./install.sh

The focused dry-run harness verified simulated Raspbian 13 emits https://download.docker.com/linux/debian trixie stable and https://download.docker.com/linux/debian/gpg, with no linux/raspbian trixie entry. It also verified simulated Raspbian 12 still emits https://download.docker.com/linux/raspbian bookworm stable and https://download.docker.com/linux/raspbian/gpg.

I also checked Docker's Debian Trixie package indexes return 200 for binary-armhf, binary-arm64, and binary-amd64.

- Description for the changelog

Use Debian APT repositories for Raspbian Trixie installs.

- A picture of a cute animal (not mandatory but encouraged)

Not included.