Check ipt options before looking for ip6t
iptables package has a function `detectIptables()` called to initialize
some local variables. Since v20.10.0, it first looks for iptables bin,
then ip6tables and finally it checks what iptables flags are available
(including -C). It early exits when ip6tables isn't available, and
doesn't execute the last check.
To remove port mappings (eg. when a container stops/dies), Docker
first checks if those NAT rules exist and then deletes them. However, in
the particular case where there's no ip6tables bin available, iptables
`-C` flag is considered unavailable and thus it looks for NAT rules by
using some substring matching. This substring matching then fails
because `iptables -t nat -S POSTROUTING` dumps rules in a slighly format
than what's expected.
For instance, here's what `iptables -t nat -S POSTROUTING` dumps:
```
-A POSTROUTING -s 172.18.0.2/32 -d 172.18.0.2/32 -p tcp -m tcp --dport 9999 -j MASQUERADE
```
And here's what Docker looks for:
```
POSTROUTING -p tcp -s 172.18.0.2 -d 172.18.0.2 --dport 9999 -j MASQUERADE
```
Because of that, those rules are considered non-existant by Docker and
thus never deleted. To fix that, this change reorders the code in
`detectIptables()`.
Fixes #42127.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>
Albin Kerouanton