Rename (Docker) deploy workflow.

This commit renames `.github/workflows/docker.yml` to `.github/workflows/deploy.yml`.

This rename is necessary to allow for this workflow to be automatically synced across all tooling repos from the exercism/org-wide-files repo.
This repo will receive a second PR soon-ish, in which we'll sync an updated version of this workflow to eliminate dependabot updates for this workflow.

We've chosen to rename the file to `deploy.yml` as that best describes its purpose: ensuring that the tooling is deployed.
The fact that we use Docker internally as our deployment mechanism is merely an implementation detail.