docker: workaround RPF=0 kernel issue w/ dummy IP

Prior to this commit, some kernels (namely 6.11.0-1015-azure), gives
the following error when IP packets ingress via the _seg_pair interface:

(pwru trace)

```
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    ip_route_input_slow
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    fib_validate_source
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    __fib_validate_source
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    ip_handle_martian_source
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    sk_skb_reason_drop(SKB_DROP_REASON_IP_RPFILTER)
0xffff95c58b7854e8 1   ksoftirqd/1:31   4026533013 0               2         0x0800 1500  60    192.168.254.2:36706->10.244.0.111:8080(tcp)    skb_release_head_state
```

even when rpf=0. This does seem to happen for instance with kernel
6.12.30 (Debian13).

This commit assigns a dummy IPv4 address, 203.0.113.1/32 which is
part of the reserved Documentation block and should never interfere
with any address, to work-around this issue.

Author: msune

docker/entrypoint.sh

@@ -138,6 +138,8 @@ if [[ "${SEG_DEV_NAME}" != "" ]]; then
 	SEG_PAIR_DEV_IFINDEX=$(ip link show ${SEG_DEV_NAME}_pair | head -n 1 | awk '{print $1}' | tr -d ':')
 	SEG_PAIR_DEV_MAC="$(ip -j link show ${SEG_DEV_NAME}_pair | jq -r '.[0].address' | tr -d ':' | sed 's/\(..\)/0x\1, /g' | sed 's/,\s*$$//')"
 
+	(ip addr show dev ${SEG_DEV_NAME}_pair | grep -q 'inet ') || ip addr add 203.0.113.1/32 dev ${SEG_DEV_NAME}_pair
+
 	sysctl -q net.ipv4.conf.${SEG_DEV_NAME}.rp_filter=0
 	sysctl -q net.ipv4.conf.${SEG_DEV_NAME}.accept_local=1
 	sysctl -q net.ipv4.conf.${SEG_DEV_NAME}_pair.rp_filter=0