Add multi-arch (amd64/arm64) container builds and parallel CI (#807)

Pull requests back-ported to release/v2.2: 801, 802, 804, 805

Restructure container builds for multi-platform support, enabling
ARM-based deployments (e.g. Apple Silicon via Docker Desktop).

Makefile:
- Replace per-container named targets with pattern rules
  (container-%, manifest-%, platform-%-{amd64,arm64},
  combine-manifest-%)
- Add parallel CI targets: platform builds push per-arch images,
  combine-manifest creates and pushes the multi-arch manifest list
- Remove legacy cruft targets (update-dcs, update-templates)

CI (release.yaml):
- Split single deploy job into build-platform-image (16 parallel
  jobs: 8 containers x 2 platforms) and combine-manifests (8 jobs,
  metadata only)
- Use native ARM runners (ubuntu-24.04-arm)

Containerfile.hf:
- Downgrade to Python 3.12 (PyTorch lacks arm64 wheels for 3.13)
- Use standard PyTorch package instead of +cpu variant (no arm64 wheels
  on the cpu index)

containers/Containerfile.hf

@@ -1,22 +1,22 @@
 
-# ----------------------------------------------------------------------------
-# Build an AI container.  This does the torch install which is huge, and I
-# like to avoid re-doing this.
-# ----------------------------------------------------------------------------
+# Torch is stable and compiles for ARM64 and AMD64 on Python 3.12
 
 FROM docker.io/fedora:42 AS ai
 
 ENV PIP_BREAK_SYSTEM_PACKAGES=1
 
-RUN dnf install -y python3.13 && \
-  alternatives --install /usr/bin/python python /usr/bin/python3.13 1 && \
+RUN dnf install -y python3.12 && \
+  alternatives --install /usr/bin/python python /usr/bin/python3.12 1 && \
   python -m ensurepip --upgrade && \
   pip3 install --no-cache-dir build wheel aiohttp && \
   pip3 install --no-cache-dir pulsar-client==3.7.0 && \
   dnf clean all
 
-RUN pip3 install torch==2.5.1+cpu \
-    --index-url https://download.pytorch.org/whl/cpu
+# This won't work on ARM
+#RUN pip3 install torch==2.5.1+cpu \
+#    --index-url https://download.pytorch.org/whl/cpu
+
+RUN pip3 install torch
 
 RUN pip3 install --no-cache-dir \
     langchain==0.3.25 langchain-core==0.3.60 langchain-huggingface==0.2.0 \