[llvm] [Github] Make Windows container use zstd (PR #167022)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Output from `skopeo inspect docker://ghcr.io/boomanaiden154/ci-windows-2022:latest | jq` (the image pushed by this workflow to my fork:
```json
{
  "Name": "ghcr.io/boomanaiden154/ci-windows-2022",
  "Digest": "sha256:36c3c6959b25c25a9496644fdc56a3a9fc6207e2ddae2f59fe0cb1fbf46b416e",
  "RepoTags": [
    "1762661533",
    "latest"
  ],
  "Created": "2025-11-09T04:12:21.4696948Z",
  "DockerVersion": "",
  "Labels": null,
  "Architecture": "amd64",
  "Os": "windows",
  "Layers": [
    "sha256:05ecc0d5d730e6d5a9a5d840fe85e2773d78b2825bdf781ad48ba107261905a4",
    "sha256:5a90d48a2ac708ac5c8a0e1520a7b38bef7116b8740a94609f931ffe3ef27450"
  ],
  "LayersData": [
    {
      "MIMEType": "application/vnd.oci.image.layer.v1.tar+zstd",
      "Digest": "sha256:05ecc0d5d730e6d5a9a5d840fe85e2773d78b2825bdf781ad48ba107261905a4",
      "Size": 113188653,
      "Annotations": null
    },
    {
      "MIMEType": "application/vnd.oci.image.layer.v1.tar+zstd",
      "Digest": "sha256:5a90d48a2ac708ac5c8a0e1520a7b38bef7116b8740a94609f931ffe3ef27450",
      "Size": 62542,
      "Annotations": null
    }
  ],
  "Env": null
}
```

I'm thinking we can eventually refactor the `push-container` composite action to use skopeo and then we can use it for both the Linux and Windows jobs. I'll leave that to a future patch though. This should save quite a bit of time because we don't actually need to load/extract the container in the push job, just recompress it.

https://github.com/llvm/llvm-project/pull/167022