[Feature] Optional input for setting repository name

👋 The Packer project has a [nightly-release](https://github.com/hashicorp/packer/blob/master/.github/workflows/nightly-release.yml) action that runs on a scheduled cron. The action executes the main CRT build workflow but fails when trying to build the docker images. See recent failure [here](https://github.com/hashicorp/packer/actions/runs/1925363414)

The failure appears to be related to REPO_NAME not getting set, which is set to `github.event.repository.name` in the main [action.yml](https://github.com/hashicorp/actions-docker-build/blob/43a393edf01f38d0fd6a0efef76217c84af17f45/action.yml#L92) file. As I learned from @mdeggies recent PRs to Packer this event object is not available for scheduled jobs. So I would like request an optional input variable for setting repo_name.

If the input is set the REPO_NAME env set [here](https://github.com/hashicorp/actions-docker-build/blob/43a393edf01f38d0fd6a0efef76217c84af17f45/action.yml#L92) would use the value of the input. If not set it should default to use `"${{github.event.repository.name}}"`.

Thanks in advance

Start with action.yml at the REPO_NAME environment assignment referenced in the issue, then review how the action declares and consumes inputs. Done means an optional repository-name input overrides REPO_NAME when provided, while scheduled runs without it retain the existing repository-name fallback.

Written by the indexing model from the issue text.