refactor: Move to explicit docker.io/* image references.

* Some runtimes (e.g. podman), require explicit registries in image URLs
 or will prompt for the user to select one, which breaks things like
 VSCode remote container rebuilds.

.github/workflows/hardware-metadata-validation.yml

@@ -29,7 +29,7 @@ jobs:
   validate-metadata:
     runs-on: ubuntu-latest
     container:
-      image: zmkfirmware/zmk-dev-arm:2.5
+      image: docker.io/zmkfirmware/zmk-dev-arm:2.5
     steps:
       - uses: actions/checkout@v2
       - name: Install dependencies