Add support for "latest" Docker Compose version

[Copilot]

This PR adds the ability to use "latest" as a value for the compose_version configuration option. This allows users to automatically get the most recent stable version of Docker Compose without having to manually update their Vagrantfile when new versions are released.

Implementation Details

• When compose_version is set to "latest", the plugin will:

  • Resolve the actual latest version by following the redirect from GitHub's latest release URL
  • Use the resolved version for downloading, installing, and symlinking Docker Compose
  • Display informative messages in the Vagrant output about the resolved version

• The implementation uses the HTTP redirect from GitHub's latest release page rather than the GitHub API to avoid rate limiting issues

Example Usage

Vagrant.configure("2") do |config|
  config.vm.box = "ubuntu/bionic64"

  config.vm.provision :docker
  config.vm.provision :docker_compose, compose_version: "latest"
end

Documentation

The README has been updated to document this new capability in the "Other configs" section.

Fixes #30.

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