fix: minor fixes for GAR deploy CircleCI jobs (#764)

This now mounts the workspace so we can load the built image from that.
Also, I fixed the image name in the `gcp-gcr/push-image` step.

This was tested successfully in
https://app.circleci.com/pipelines/github/mozilla-services/autopush-rs/2873/workflows/9adebe2f-13e8-4e5d-a853-e78b7f6ca368

Author: emaydeck-mozilla

.circleci/config.yml

@@ -436,6 +436,11 @@ jobs:
           use_oidc: true
           workload_identity_pool_id: GCP_OIDC_WIP_ID
           workload_identity_pool_provider_id: GCP_OIDC_WIP_PROVIDER_ID
+      - attach_workspace:
+          at: /tmp/cache
+      - run:
+          name: Restore Docker image cache
+          command: docker load -i /tmp/cache/docker.tar
       - run:
           name: Tag image
           command: |
@@ -451,7 +456,7 @@ jobs:
       # push-image parameters:
       # https://circleci.com/developer/orbs/orb/circleci/gcp-gcr#commands-push-image
       - gcp-gcr/push-image:
-          image: $GAR_IMAGE
+          image: "${GCP_GAR_REPO}/<<parameters.image>>"
           google-project-id: GCP_GAR_PROJECT_ID
           registry-url: <<parameters.registry-url>>
           tag: $GAR_TAG,latest