arc: add snapshot-init wrapper for testnet pruned bootstrap

Shell entrypoint wraps arc-snapshots so compose can optionally override EL/CL
snapshot URLs via ARC_SNAPSHOT_* env vars while defaulting to latest from
snapshots.arc.network on fresh volumes.

arc/arc-snapshot-init.Dockerfile

@@ -0,0 +1,12 @@
+# One-shot Arc snapshot downloader for compose init containers.
+# Wraps arc-snapshots with a shell entrypoint so operators can optionally
+# override EL/CL snapshot URLs via environment variables.
+ARG ARC_EXECUTION_IMAGE=docker.cloudsmith.io/circle/arc-network/arc-execution:0.7.2
+FROM ${ARC_EXECUTION_IMAGE} AS arc-bin
+FROM debian:bookworm-slim
+RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates libssl3 zlib1g \
+    && rm -rf /var/lib/apt/lists/*
+COPY --from=arc-bin /usr/local/bin/arc-snapshots /usr/local/bin/arc-snapshots
+COPY scripts/snapshot-init.sh /usr/local/bin/snapshot-init.sh
+RUN chmod +x /usr/local/bin/snapshot-init.sh
+ENTRYPOINT ["/usr/local/bin/snapshot-init.sh"]