ci: replace registry container action reference

Use a local Docker container action so the runner still exercises its container-action mounts and environment without a docker:// workflow reference.

Signed-off-by: Quentin Laplanche <quentin.laplanche@docker.com>

Co-authored-by: Codex <noreply@openai.com>

.github/actions/docker-pull/Dockerfile

@@ -0,0 +1,3 @@
+FROM docker:29.3@sha256:4d90f1f6c400315c2dba96d3ec93c01e64198395cbba04f79d12adce4f737029
+
+ENTRYPOINT ["docker"]

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

@@ -0,0 +1,15 @@
+name: Pull Docker image
+
+description: Pull an image from a Docker container action
+
+inputs:
+  image:
+    description: Image reference to pull
+    required: true
+
+runs:
+  using: docker
+  image: Dockerfile
+  args:
+    - pull
+    - ${{ inputs.image }}