ci: convert OCI pipeline to parity-lib nix2container builder

The payload (pure-stdlib bridge.py + a stock CPython closure) is fully
Nix-expressible, so this is NOT an escape-hatch/buildkit repo: both arches
build on emmett (amd64 native + arm64 pkgsCross of stock python3 from the
binary cache) with no buildkit, qemu, docker daemon, or howard pin.

Replace the partial amd64-only scaffold with parity-lib's
mkNix2ContainerPublish, completing the arm64 leg + multi-arch index. The
per-arch nix2container image derivations are kept verbatim; stage/publish/
publish-index/publish/push-staged now come from the shared builder so CI and
local invoke identical code. Thin .woodpecker.yaml to a single
nix run .#publish; retire the buildx/remote-builder steps.

The Dockerfile is now unused (the cutover drops it) but kept in-tree so the
server-side hadolint pre-receive hook does not crash on a file deletion.

Refs cluster #192, emmett#44.

flake.lock

@@ -18,6 +18,24 @@
         "type": "github"
       }
     },
+    "flake-utils_2": {
+      "inputs": {
+        "systems": "systems_2"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "fleet": {
       "inputs": {
         "nixpkgs": "nixpkgs",
@@ -72,6 +90,69 @@
         "url": "https://git.oleks.space/oleks/fleet-pins"
       }
     },
+    "fleet_2": {
+      "inputs": {
+        "nixpkgs": "nixpkgs_2",
+        "nixpkgs-armer": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-bim": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-ci": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-emmett": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-howard": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-mermaid": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-mermaid-gpu": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-micron": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ],
+        "nixpkgs-projects": [
+          "parity",
+          "fleet",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1779533061,
+        "narHash": "sha256-orWNYXtYURhEj3X4+xGMAhaEcKRvwXqTtJ8x2jV/M+Q=",
+        "ref": "refs/heads/main",
+        "rev": "b818e345ec4470e4b3e335bd2f864183c512116d",
+        "revCount": 13,
+        "type": "git",
+        "url": "https://git.oleks.space/oleks/fleet-pins"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://git.oleks.space/oleks/fleet-pins"
+      }
+    },
     "nix2container": {
       "inputs": {
         "nixpkgs": [
@@ -108,6 +189,46 @@
         "type": "github"
       }
     },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1777268161,
+        "narHash": "sha256-bxrdOn8SCOv8tN4JbTF/TXq7kjo9ag4M+C8yzzIRYbE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "1c3fe55ad329cbcb28471bb30f05c9827f724c76",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "1c3fe55ad329cbcb28471bb30f05c9827f724c76",
+        "type": "github"
+      }
+    },
+    "parity": {
+      "inputs": {
+        "flake-utils": "flake-utils_2",
+        "fleet": "fleet_2",
+        "nixpkgs": [
+          "parity",
+          "fleet",
+          "nixpkgs-ci"
+        ]
+      },
+      "locked": {
+        "lastModified": 1780379798,
+        "narHash": "sha256-vm88bZ2O/KHb14dUPlpuSMPzQlwDRwiTfZrZUMpo1Pw=",
+        "ref": "refs/heads/main",
+        "rev": "af64a8ea4c537f6b5e9f2bee0fbddc59648f7d32",
+        "revCount": 7,
+        "type": "git",
+        "url": "https://git.oleks.space/oleks/parity-lib"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://git.oleks.space/oleks/parity-lib"
+      }
+    },
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
@@ -116,7 +237,8 @@
         "nixpkgs": [
           "fleet",
           "nixpkgs-projects"
-        ]
+        ],
+        "parity": "parity"
       }
     },
     "systems": {
@@ -133,6 +255,21 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",