hack: dependabot ls command

[jakecoffman]

This adds the dependabot ls command which outputs the parsed dependencies, and then stops the containers:

$ dependabot ls go_modules dependabot/cli 2> /dev/null
[
  {
    "name": "github.com/MakeNowJust/heredoc",
    "requirements": [
      {
        "file": "go.mod",
        "groups": [],
        "requirement": "v1.0.0",
        "source": {
          "source": "github.com/MakeNowJust/heredoc",
          "type": "default"
        }
      }
    ],
    "version": "1.0.0"
  },
... (and so on for a while)

The reason this is labelled as a hack is the Dependabot Updater wants to continue with the update, but I have the CLI cancel the Context which terminates the Docker containers.

A more graceful solution would involve telling dependabot-core directly that we want to stop after the dependencies are gathered.

For now this works as a POC.

In the console command above, I've redirected stderr to /dev/null as by default the Dependabot CLI outputs logs to stderr.