app-admin/toolbox: fix access to private toolbox image on GCR

Use docker-credential-gcr to generate credentials string for
the containers from gcr.io registry and mirrors to authorize
access to private images.

BUG=b/211904749
TEST=presubmit
RELEASE_NOTE=Fix access to private toolbox images hosted on GCR.

Change-Id: Iaf275ec47ef01eb3fc3428669ebc94c5244aa90f
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/26860
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Varsha Teratipally <teratipally@google.com>
Reviewed-by: Rayan Dasoriya <dasoriya@google.com>
3 files changed