Local build path: pinned alt-tags, manual-only workflow, README build docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

recipes/recipe.yml

@@ -12,6 +12,11 @@ description: Bazzite DX NVIDIA (open, KDE) with my first-boot Flatpak set, signe
 base-image: ghcr.io/ublue-os/bazzite-dx-nvidia
 # Pinned to Fedora 44. Bump deliberately (stable-45, ...) when ready to jump majors.
 image-version: stable-44
+# Deterministic tags regardless of where the build runs (local builds would
+# otherwise only produce a local-<version> tag; rebase refs rely on :latest).
+alt-tags:
+  - latest
+  - stable-44
 
 modules:
   - type: default-flatpaks