Use docker to run helper servers in e2e

Makefile

@@ -181,6 +181,7 @@ test: $(BUILD_DIRS)
 
 test-tools:
 	@docker build -t $(REGISTRY)/test/test-sshd _test_tools/sshd
+	@docker build -t $(REGISTRY)/test/test-ncsvr _test_tools/ncsvr
 
 # Help set up multi-arch build tools.  This assumes you have the tools
 # installed.  If you already have a buildx builder available, you don't need

_test_tools/ncsvr/Dockerfile

@@ -0,0 +1,25 @@
+# Stolen from https://github.com/linuxkit/linuxkit/tree/master/pkg/sshd/
+
+FROM alpine AS base
+
+RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
+RUN apk add --no-cache --initdb -p /out \
+    alpine-baselayout \
+    apk-tools \
+    busybox \
+    util-linux \
+    netcat-openbsd \
+    && true
+
+###############
+
+FROM scratch
+
+ENTRYPOINT []
+WORKDIR /
+
+COPY --from=base /out/ /
+
+COPY ncsvr.sh /
+
+ENTRYPOINT ["/ncsvr.sh"]

_test_tools/ncsvr/README.md

@@ -0,0 +1,38 @@
+# A simple server for tests
+
+DO NOT USE THIS FOR ANYTHING BUT TESTING!!!
+
+## How to use it
+
+Build yourself a test image.  We use example.com so you can't accidentally push
+it.
+
+```
+$ docker build -t example.com/test/test-ncvsr .
+...lots of output...
+Successfully tagged example.com/test/test-ncsvr:latest
+```
+
+Run it.
+
+```
+$ docker run -d example.com/test/test-ncsvr 9376 "echo hello"
+6d05b4111b03c66907031e3cd7587763f0e4fab6c50fac33c4a8284732b448ae
+```
+
+Find your IP.
+
+```
+$ docker inspect 6d05b4111b03c66907031e3cd7587763f0e4fab6c50fac33c4a8284732b448ae | jq -r .[0].NetworkSettings.IPAddress
+192.168.1.2
+```
+
+Connect to it.
+
+```
+$ echo "" | nc 192.168.9.2 9376
+hello
+```
+
+If you want to know how many times it was accessed, mount a file on
+/var/log/hits.  This will log one line per hit.

_test_tools/ncsvr/ncsvr.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+if [ -z "$1" -o -z "$2" ]; then
+    echo "usage: $0 <port> <shell-command>"
+    exit 1
+fi
+
+while true; do
+    sh -c "$2" | nc -l -p "$1" -N -w0 >/dev/null
+    date >> /var/log/hits
+done