feat: NVSHAS-9502 create standalone Dockerfile

1. Provide cross platform/standalone Dockerfile
2. Release.yml to publish SLSA-capable artifacts
3. Provide build target, test-image, build-image and push-image to sync
   with rancher.
4. Upgrade to BCI 15.6

.github/workflows/release.yml

@@ -59,8 +59,8 @@ jobs:
 
         public-registry: docker.io
         public-repo: rancher
-        public-username: ${{ env.DOCKER_USERNAME }}
-        public-password: ${{ env.DOCKER_PASSWORD }}
+        public-username: ${{ env.RANCHER_DOCKER_USERNAME }}
+        public-password: ${{ env.RANCHER_DOCKER_PASSWORD }}
 
         prime-registry: ${{ env.PRIME_REGISTRY }}
         prime-repo: rancher