forwarder: propagate source IP

[AkihiroSuda]

Fix:

• gvisor-tap-vsock port driver does not propagate the src IP rootless-containers/rootlesskit#573

CONTAINERD_ROOTLESS_ROOTLESSKIT_NET=gvisor-tap-vsock \
CONTAINERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=gvisor-tap-vsock \
containerd-rootless.sh 

nerdctl run -d -p 8080:80 nginx:alpine
curl HOST_IP:8080
nerdctl logs nginx

When the port driver is set to gvisor-tap-vsock, the source IP is always set to 10.0.2.1.
The correct IP is propagated when the port driver is set to builtin or slirp4netns (w/ NET=slirp4netns too).

Note: used Claude Code

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[gbraad]

@cfergeau WDYT? We need this for containerd rootless use

[cfergeau]

@cfergeau WDYT? We need this for containerd rootless use

This was marked as "work in progress" until recently so I did not look closely, but no objection to the general idea.