Print ghcr docker pull during build/test (#114510)

To make debugging easier to external devs

Test plan: Copy and run command from [`Use the following to pull public copy of the image`](https://github.com/pytorch/pytorch/actions/runs/7012511180/job/19077533416?pr=114510#step:6:9):
```
docker pull ghcr.io/pytorch/ci-image:pytorch-linux-jammy-py3.8-gcc11-0d0042fd2e432ea07301ad6f6a474d36a581f0dc

```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/114510
Approved by: https://github.com/atalman, https://github.com/huydhn
diff --git a/.github/workflows/_linux-build.yml b/.github/workflows/_linux-build.yml
index 895507f..9a88ed7 100644
--- a/.github/workflows/_linux-build.yml
+++ b/.github/workflows/_linux-build.yml
@@ -93,6 +93,15 @@
         with:
           docker-image-name: ${{ inputs.docker-image-name }}
 
+      - name: Use following to pull public copy of the image
+        id: print-ghcr-mirror
+        env:
+          ECR_DOCKER_IMAGE: ${{ steps.calculate-docker-image.outputs.docker-image }}
+        shell: bash
+        run: |
+          tag=${ECR_DOCKER_IMAGE##*/}
+          echo "docker pull ghcr.io/pytorch/ci-image:${tag/:/-}"
+
       - name: Pull docker image
         uses: pytorch/test-infra/.github/actions/pull-docker-image@main
         with:
diff --git a/.github/workflows/_linux-test.yml b/.github/workflows/_linux-test.yml
index 685c055..4bd0e38 100644
--- a/.github/workflows/_linux-test.yml
+++ b/.github/workflows/_linux-test.yml
@@ -77,6 +77,15 @@
         with:
           docker-image-name: ${{ inputs.docker-image }}
 
+      - name: Use following to pull public copy of the image
+        id: print-ghcr-mirror
+        env:
+          ECR_DOCKER_IMAGE: ${{ steps.calculate-docker-image.outputs.docker-image }}
+        shell: bash
+        run: |
+          tag=${ECR_DOCKER_IMAGE##*/}
+          echo "docker pull ghcr.io/pytorch/ci-image:${tag/:/-}"
+
       - name: Pull docker image
         uses: pytorch/test-infra/.github/actions/pull-docker-image@main
         with: