fix: add ecr authentication (#129)

Issue #, if available:

*Description of changes:*

use aws-actions/amazon-ecr-login@v1 to configure credentials / get
authorization token to Amazon ECR.

*Testing done:*

act, local repo.


- [x] I've reviewed the guidance in CONTRIBUTING.md


#### License Acceptance

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Signed-off-by: Gavin Inglis <[email protected]>

.github/workflows/rootfs.yaml

@@ -27,6 +27,8 @@ jobs:
           aws-region: ${{ secrets.REGION }}
           role-to-assume: ${{ secrets.ROLE }}
           role-session-name: rootfs-ecr-image-upload-session
+      - name: Login to Amazon ECR
+        uses: aws-actions/amazon-ecr-login@v1
       - name: checkout repo
         uses: actions/checkout@v3
         with: