Official ContainerSSH image lacks linux/arm64 support

## Problem

The official `containerssh/containerssh` image on Docker Hub appears to be published as `linux/amd64` only, with no `linux/arm64` variant.

As of May 11, 2026, Docker Hub shows single-arch `amd64` images for the published tags, and the current image build source in `ContainerSSH/images` hardcodes the `amd64` release artifact download:

https://github.com/ContainerSSH/images/blob/main/containerssh/Dockerfile

Specifically, it downloads:

`containerssh_${CONTAINERSSH_VERSION/v/}_linux_amd64.tar.gz`

## Expected behavior

The official ContainerSSH image should be published as a multi-arch image including at least:

- `linux/amd64`
- `linux/arm64`

## Notes

The main `ContainerSSH/ContainerSSH` repo already builds `arm64` release binaries, so this looks like an image build/publish gap rather than a code compatibility issue.

Related PR:
https://github.com/ContainerSSH/images/pull/5