Multi arch cpu (#746)

Encountered a similar problem as in
https://github.com/mostlygeek/llama-swap/issues/709 but in my case I
only needed the :cpu version.

So decided to add the github action to build arm64 combined with the
amd64 version on the same :cpu tag. Already tested it from this fork:
ghcr.io/rhtenhove/llama-swap:cpu and it works perfectly fine.

Adding GPU support is a whole other beast, needing quite a bit more work
and isn't something I can test.

docker/llama-swap.Containerfile

@@ -3,6 +3,9 @@ ARG BASE_TAG=server-cuda
 FROM ${BASE_IMAGE}:${BASE_TAG}
 
 # has to be after the FROM
+# TARGETARCH is auto-set by `docker buildx build --platform …` (amd64/arm64);
+# falls back to amd64 when an older `docker build` runs without buildx.
+ARG TARGETARCH=amd64
 ARG LS_VER=170
 ARG LS_REPO=mostlygeek/llama-swap
 
@@ -34,9 +37,9 @@ WORKDIR /app
 ENV PATH="/app:${PATH}"
 
 RUN \
-    curl -LO "https://github.com/${LS_REPO}/releases/download/v${LS_VER}/llama-swap_${LS_VER}_linux_amd64.tar.gz" && \
-    tar -zxf "llama-swap_${LS_VER}_linux_amd64.tar.gz" && \
-    rm "llama-swap_${LS_VER}_linux_amd64.tar.gz"
+    curl -LO "https://github.com/${LS_REPO}/releases/download/v${LS_VER}/llama-swap_${LS_VER}_linux_${TARGETARCH}.tar.gz" && \
+    tar -zxf "llama-swap_${LS_VER}_linux_${TARGETARCH}.tar.gz" && \
+    rm "llama-swap_${LS_VER}_linux_${TARGETARCH}.tar.gz"
 
 COPY --chown=$UID:$GID config.example.yaml /app/config.yaml