Add test that runs runsc do inside a non-gVisor container.

This is used in contexts such as Dangerzone:
https://gvisor.dev/blog/2024/09/23/safe-ride-into-the-dangerzone/

Updates issue #10944.

PiperOrigin-RevId: 682454284

images/basic/integrationtest/Dockerfile

@@ -4,7 +4,8 @@ WORKDIR /root
 COPY . .
 RUN chmod +x *.sh
 
-RUN apt-get update && apt-get install -y gcc iputils-ping iproute2
+RUN apt-get update && apt-get install -y \
+    build-essential iputils-ping iproute2 iptables
 
 # Compilation Steps.
 RUN gcc -O2 -o test_copy_up test_copy_up.c
@@ -12,4 +13,12 @@ RUN gcc -O2 -o test_rewinddir test_rewinddir.c
 RUN gcc -O2 -o link_test link_test.c
 RUN gcc -O2 -o test_sticky test_sticky.c
 RUN gcc -O2 -o host_fd host_fd.c
-RUN gcc -O2 -o host_connect host_connect.c
+RUN gcc -O2 -o host_connect host_connect.c
+
+# Add nonprivileged regular user named "nonroot".
+RUN groupadd --gid 1337 nonroot && \
+    useradd --uid 1337 --gid 1337 \
+        --create-home \
+        --shell $(which bash) \
+        --password '' \
+        nonroot