tools: Add a few more packages to the Alpine base image

- iproute2 provides a more complete suite of 'ip' utilities than
  the busybox implementation. Useful, for debugging.
- iperf3 for network testing
- tcpdump for network debugging
- expect is used in some tests. Adding it to the base image should
  make it easier to create a LinuxKit image to self-host CI.

The new version of the base package is:
linuxkit/alpine:3d57ded3edd54e616210cf0c17e3bc15eed9d03a

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

tools/alpine/packages

@@ -25,6 +25,7 @@ dosfstools
 e2fsprogs
 e2fsprogs-extra
 elfutils-dev
+expect
 flex
 gcc
 git
@@ -34,6 +35,8 @@ go
 gummiboot
 hvtools
 installkernel
+iperf3
+iproute2
 iptables
 jq
 kmod
@@ -71,6 +74,7 @@ strace
 swig
 syslinux
 tar
+tcpdump
 tini
 tzdata
 util-linux