mkNix2ContainerPublish: add impureBuild + indexMovesLatest modes (cluster #205)

impureBuild: build the consumer's flake attr at run time via
nix build --impure --sandbox false, instead of embedding the image
closure as an eval-time build dep — required for images that fetch
private artifacts with a token at build time (ii-agent). indexMovesLatest:
publish-index also moves :latest, for repos that publish each arch on a
separate CI agent and converge in a final index-only step. Both opt-in,
default-off; existing consumers unchanged. Verified by eval in both modes.

lib/builders.nix

@@ -679,11 +679,44 @@ let
       registryHost ? "git.oleks.space",
       registryOwner ? "oleks",
       passEntry ? "infra/gitea/personal_access_token_packages_rw",
+      # IMPURE-BUILD mode (cluster #205). Default: each `images.<arch>` carries a
+      # `copyTo` derivation referenced as an EVAL-TIME store path (a build dep of
+      # the app) — correct for a pure image. But some images are built IMPURELY
+      # (need a registry token / network / sandbox off at build time, e.g.
+      # ii-agent fetching private wheels), so the closure CANNOT be a build dep
+      # of the app. When `impureBuild = true`, each `images.<arch>` instead
+      # carries `attr` (a flake attribute on the CONSUMER's own flake, e.g.
+      # "frontend-image"), and stage/publish/verify-digest build it at RUN time
+      # via `nix build --impure --option sandbox false ".#<attr>.copyTo"`.
+      impureBuild ? false,
+      # When true, `publish-index` ALSO moves `:latest` (a dev-guarded digest
+      # copy of `:VERSION`) after assembling the index. Default off: `:latest`
+      # is owned by the `publish` app. Repos that publish each arch on a SEPARATE
+      # CI agent and converge in a final index-only step (ii-agent) need the
+      # latest-move to live in `publish-index`. cluster #205.
+      indexMovesLatest ? false,
     }:
     let
       head = preamble { inherit registryHost registryOwner passEntry; };
       localArches = lib.attrNames images;
 
+      # Resolve one arch's nix2container copy-to wrapper into shell var $1.
+      # Pure: an eval-time store path (build dep of the app). Impure (cluster
+      # #205): build the consumer's own flake attr at RUN time and read the path
+      # back — the impure layers (token/network/sandbox-off) never become a build
+      # dep of the app, which would otherwise fail under the pure sandbox.
+      resolveCopyTo =
+        arch: var:
+        if impureBuild then
+          ''
+            IMAGE_TAG="$VERSION-${arch}" nix build --impure --no-link \
+              --option sandbox false ".#${images.${arch}.attr}.copyTo" >/dev/null
+            ${var}="$(IMAGE_TAG="$VERSION-${arch}" nix eval --impure --raw \
+              ".#${images.${arch}.attr}.copyTo")"
+          ''
+        else
+          "${var}=${lib.escapeShellArg "${images.${arch}.copyTo}"}";
+
       # per-arch stage: realise the image's copy-to closure locally (no push).
       mkArchStage =
         arch:
@@ -696,7 +729,7 @@ let
             echo "→ staging ${imageName}:$VERSION-${arch} (build closure only, no push)"
             d="$(parity_stage_reset ${lib.escapeShellArg arch})"
             # Realise the nix2container copyTo wrapper (its closure is the image).
-            out=${lib.escapeShellArg "${images.${arch}.copyTo}"}
+            ${resolveCopyTo arch "out"}
             printf 'oci %s %s %s\n' ${lib.escapeShellArg imageName} "$VERSION" "${arch}" >"$d/.parity-meta"
             printf '%s\n' "$out" >"$d/copy-to-path"
             echo "   staged copyTo: $out"
@@ -715,7 +748,8 @@ let
       digestArch = arch: ''
         echo "→ ${imageName}:$VERSION-${arch} (local build, no registry contact)"
         ocidir="$(mktemp -d)"
-        ${lib.escapeShellArg "${images.${arch}.copyTo}"}/bin/copy-to "oci:$ocidir:${arch}" >/dev/null
+        ${resolveCopyTo arch "copyto"}
+        "$copyto/bin/copy-to" "oci:$ocidir:${arch}" >/dev/null
         digest="$(skopeo manifest-digest "$ocidir/blobs/sha256/$(
           jq -r '.manifests[0].digest | sub("sha256:";"")' "$ocidir/index.json"
         )")"
@@ -758,7 +792,10 @@ let
             tok="$(parity_resolve_token)"
             parity_registry_preflight
             creds="$PARITY_REGISTRY_OWNER:$tok"
-            ${lib.escapeShellArg "${images.${arch}.copyTo}"}/bin/copy-to \
+            # Read the copy-to path the stage step just wrote (works for both
+            # pure and impure modes — stage already realised/built the closure).
+            copyto="$(cat "$PARITY_STAGE_DIR/${arch}/copy-to-path")"
+            "$copyto/bin/copy-to" \
               --dest-creds "$creds" \
               "docker://${imageName}:$VERSION-${arch}"
             echo "Pushed ${imageName}:$VERSION-${arch}"
@@ -809,6 +846,18 @@ let
           done
           regctl index create "${imageName}:$VERSION" "''${refs[@]/#/--ref=}"
           echo "Assembled index ${imageName}:$VERSION"
+          ${lib.optionalString indexMovesLatest ''
+            # Repos that converge per-arch legs in this index-only step own the
+            # ':latest' move here (rather than in `publish`). Dev-guarded; a pure
+            # digest copy of the just-assembled ':VERSION', done LAST.
+            case "$VERSION" in
+              *dev*) echo "  dev version $VERSION — refusing to move :latest" ;;
+              *)
+                regctl image copy "${imageName}:$VERSION" "${imageName}:latest"
+                echo "→ ${imageName}:$VERSION -> :latest (digest copy)"
+                ;;
+            esac
+          ''}
         '';
       };