fix(ci): isolate agent smoke in namespace-capable helper

scripts/ci/linux-agent-smoke.Dockerfile

@@ -0,0 +1,23 @@
+FROM debian:bookworm-slim
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+      bash ca-certificates coreutils curl iproute2 ipset iptables netbase nftables python3 util-linux \
+    && rm -rf /var/lib/apt/lists/*
+# The harness masks /etc. Debian's public xtables links normally traverse
+# /etc/alternatives, so resolve them while that directory is still visible.
+RUN set -eu; for tool in iptables iptables-save iptables-restore ip6tables ip6tables-save ip6tables-restore; do \
+      binary="$(readlink -f "$(command -v "$tool")")"; \
+      test -x "$binary"; \
+      case "$binary" in /etc/*) exit 1;; esac; \
+      ln -sf "$binary" "/usr/sbin/$tool"; \
+      test "$(readlink -f "/usr/sbin/$tool")" = "$binary"; \
+    done
+WORKDIR /work
+# Public protocol/service name databases only; never copy the runner's /etc.
+RUN mkdir helper-etc && cp /etc/protocols /etc/services helper-etc/
+# COPY makes this work with a remote Docker daemon; no runner path is mounted.
+COPY apps/api/src/agent-scripts/evofw-firewall.sh apps/api/src/agent-scripts/evofw-firewall.sh
+COPY scripts/ci/linux-netns-smoke.sh scripts/ci/linux-agent-fixture.py scripts/ci/linux-agent-fixture.test.py scripts/ci/linux-agent-container-entry.sh scripts/ci/
+RUN bash -n scripts/ci/linux-netns-smoke.sh scripts/ci/linux-agent-container-entry.sh \
+    && python3 -B scripts/ci/linux-agent-fixture.test.py
+ENTRYPOINT ["bash", "scripts/ci/linux-agent-container-entry.sh"]