workflows/docker: improve job names.

This makes it a bit more obvious in various GitHub UIs that don't
display the workflow name and more consistent with `tests.yml`.

Author: MikeMcQuaid

.github/workflows/docker.yml

@@ -16,6 +16,7 @@ permissions:
 jobs:
   ubuntu:
     if: github.repository_owner == 'Homebrew'
+    name: docker (Ubuntu ${{ matrix.version }})
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false