Shieldwall does not handle FORWARD chain

Hey and thanks for creating awesome project!

I'm having some difficulty shieldwalling my docker containers, I noticed that Shieldwall doesn't add an entry to FORWARD, so it doesn't get captured. Is this a bug?

```
# iptables -n -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED
SHIELDWALL all -- 0.0.0.0/0 0.0.0.0/0
LOGNDROP all -- 0.0.0.0/0 0.0.0.0/0

Chain FORWARD (policy DROP)
target prot opt source destination
DOCKER-USER all -- 0.0.0.0/0 0.0.0.0/0
DOCKER-ISOLATION-STAGE-1 all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED
DOCKER all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 ctstate RELATED,ESTABLISHED
DOCKER all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Chain DOCKER (2 references)
target prot opt source destination
ACCEPT tcp -- 0.0.0.0/0 172.18.0.2 tcp dpt:443
ACCEPT tcp -- 0.0.0.0/0 172.18.0.2 tcp dpt:81

Chain DOCKER-ISOLATION-STAGE-1 (1 references)
target prot opt source destination
DOCKER-ISOLATION-STAGE-2 all -- 0.0.0.0/0 0.0.0.0/0
DOCKER-ISOLATION-STAGE-2 all -- 0.0.0.0/0 0.0.0.0/0
RETURN all -- 0.0.0.0/0 0.0.0.0/0

Chain DOCKER-ISOLATION-STAGE-2 (2 references)
target prot opt source destination
DROP all -- 0.0.0.0/0 0.0.0.0/0
DROP all -- 0.0.0.0/0 0.0.0.0/0
RETURN all -- 0.0.0.0/0 0.0.0.0/0

Chain DOCKER-USER (1 references)
target prot opt source destination
DROP tcp -- !127.0.0.1 0.0.0.0/0 tcp dpt:81
RETURN all -- 0.0.0.0/0 0.0.0.0/0

Chain LOGNDROP (1 references)
target prot opt source destination
LOG all -- 0.0.0.0/0 0.0.0.0/0 limit: avg 10/min burst 5 LOG flags 0 level 4 prefix "shieldwall-dropped: "
DROP all -- 0.0.0.0/0 0.0.0.0/0

Chain SHIELDWALL (1 references)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT tcp -- 173.245.48.0/20 0.0.0.0/0 tcp dpt:443
[TRUNCATED - Cloudflare]
ACCEPT tcp -- 127.0.0.1 0.0.0.0/0 tcp dpts:1:65535
ACCEPT udp -- 127.0.0.1 0.0.0.0/0 udp dpts:1:65535
```

Thanks!