Refactor Docker registry login step to use environment variables for credentials

.gitea/workflows/create-image.yaml

@@ -57,11 +57,11 @@ jobs:
           time curl -v https://${{ env.DOCKER_REGISTRY_DOMAIN }}/v2/ || true
           
       - name: Login to the Container registry
-        uses: docker/login-action@v3
-        with:
-          registry: ${{ env.DOCKER_REGISTRY_DOMAIN }}
-          username: ${{ gitea.actor }}
-          password: ${{ secrets.REGISTRY_TOKEN }}
+        env:
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+          PIPELINES_USERNAME: ${{ secrets.PIPELINES_USERNAME }}
+        run: |
+          echo "${REGISTRY_TOKEN}" | docker login "${{ env.DOCKER_REGISTRY_DOMAIN }}" -u "${PIPELINES_USERNAME}" --password-stdin
           
       - name: Get Unity Version
         if: ${{ github.event.inputs.unity_version == '' }}