docker compose fallback to the legacy docker-compose

[veyselsahin]

On Ubuntu 24.04 with the docker.io package, docker compose (Compose v2
plugin) is not installed by default, but the legacy docker-compose
binary is available and works.

The current install.sh assumes that if docker compose version fails,
Compose is not installed at all, and unconditionally tries to install
the docker-compose-plugin. This can fail in two ways:

• Ubuntu repos may not have docker-compose-plugin.
• Adding Docker's apt repo causes docker-buildx-plugin to conflict with
  Ubuntu's docker-buildx from docker.io.

Even if the install succeeds, final user instructions always print
docker compose ..., which fails on systems that only have the legacy
binary.

Fix:

• Introduce a sudo-aware COMPOSE_CMD that prefers $DOCKER_CMD compose
  but falls back to docker-compose if available.
• Use $COMPOSE_CMD for the plugin check and in final instructions.

This avoids unnecessary installs, prevents package conflicts, and makes
the installer work on both Docker's official packages and Ubuntu's
docker.io defaults.

[lukemarsden]

Thanks for the PR! I left one comment