Allow deploying to multiple environments

[NickVolynkin]

Using "$DOKKU_APP_NAME" as the git remote name enables deploying to multiple environments from a single GitHub runner.

Otherwise, the action fails to set up a new remote with the following error:https://github.com/idoberko2/dokku-deploy-github-action/pull/13/files

 fatal: remote deploy already exists.

.. but proceeds to deploy code to the wrong remote.