fix(ci): use DOCKER_USERNAME/DOCKER_PASSWORD for identity docker push

Match the secret names pedalboard's build-services workflow uses (these
are org-level secrets on AudiusProject) instead of the CircleCI-era
DOCKERHUB_USERNAME / DOCKERHUB_PASS, which aren't configured as GitHub
Actions secrets.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: dylanjeffers

.github/workflows/identity.yml

@@ -345,8 +345,8 @@ jobs:
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_PASS }}
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
 
       - name: Build and push identity-service image
         uses: docker/build-push-action@v6