Give the possibility of defining custom dotfiles when setting up the devcontainer

[arnaupv]

Thanks to the following feature: https://github.com/devcontainers/cli/pull/362 we can update the plugin for accepting custom dotfiles when setting up the devcontainer.

{
  "arnaupv/nvim-devcontainer-cli",
  opts = {
    dotfiles = "https://github.com/arnaupv/dotfiles"
  }
},  

Once the feature is defined, there will be certain part of the code: https://github.com/arnaupv/nvim-devcontainer-cli/blob/main/bin/spawn_devcontainer.sh#L26