Add IPv6 overlay addressing documentation (#667)

src/pages/get-started/cli.mdx

@@ -97,7 +97,7 @@ The command will check if the peer is logged in and connect to the management se
       --disable-client-routes               Disable client routes. If enabled, the client won't process client routes received from the management service.
       --disable-dns                         Disable DNS. If enabled, the client won't configure DNS settings.
       --disable-firewall                    Disable firewall configuration. If enabled, the client won't modify firewall rules.
-      --disable-ipv6                        Disable IPv6 overlay. If enabled, the client won't request or use an IPv6 overlay address.
+      --disable-ipv6                        Disables IPv6 overlay addressing. The client will not receive an IPv6 address or accept IPv6 traffic from peers. (v0.68.0+)
       --disable-server-routes               Disable server routes. If enabled, the client won't act as a router for server routes received from the management service.
       --disable-ssh-auth                    Disable SSH JWT authentication. If enabled, any peer with network access can connect without user authentication.
       --dns-resolver-address string         Sets a custom address for NetBird's local DNS resolver. If set, the agent won't attempt to discover the best IP and port to listen on. An empty string "" clears the previous configuration. E.g. --dns-resolver-address 127.0.0.1:5053 or --dns-resolver-address ""

src/pages/get-started/install/docker.mdx

@@ -21,6 +21,29 @@ docker run --rm --name PEER_NAME --hostname PEER_NAME --cap-add=NET_ADMIN --cap-
 
 See [Docker example](/use-cases/examples#net-bird-client-in-docker) for details.
 
+### IPv6 overlay
+
+If you are using [IPv6 overlay addressing](/manage/settings/ipv6) and the container is not running with `--network=host`, add the IPv6 sysctls so the kernel forwards v6 traffic inside the container:
+
+```bash
+docker run ... \
+  --sysctl net.ipv6.conf.all.disable_ipv6=0 \
+  --sysctl net.ipv6.conf.all.forwarding=1 \
+  netbirdio/netbird:latest
+```
+
+Or in Docker Compose:
+
+```yaml
+services:
+  netbird-client:
+      sysctls:
+          - net.ipv6.conf.all.disable_ipv6=0
+          - net.ipv6.conf.all.forwarding=1
+```
+
+With `network_mode: host` the container inherits the host's sysctls, so the flags can be dropped as long as the host has IPv6 forwarding enabled.
+
 ### Troubleshooting
 1. If you are using self-hosted version and haven't specified `--management-url`, the client app will use the default URL
 which is ```https://api.netbird.io:443```.