ci: fix configure docker action inputs

The options used were wrong in two ways: firstly they were not enforced
as a "choice" (i.e. invalid input valudes could be provided without
error) and one of the options was listed as `gh` when we passed it as
`gha` from ci.yml.

"Fix" this by removing the choice altogether but sanity-testing the
input value against an expected list using a GHA "warning" to notify of
unknown inputs.

.github/actions/configure-docker/action.yml

@@ -4,12 +4,21 @@ inputs:
   cache-provider:
     description: 'gha or cirrus cache provider'
     required: true
-    options:
-    - gh
-    - cirrus
 runs:
   using: 'composite'
   steps:
+    - name: Check inputs
+      shell: bash
+      run: |
+        # We expect only gha or cirrus as inputs to cache-provider
+        case "${{ inputs.cache-provider }}" in
+          gha|cirrus)
+            ;;
+          *)
+            echo "::warning title=Unknown input to configure docker action::Provided value was ${{ inputs.cache-provider }}"
+            ;;
+        esac
+
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v3
       with: