Add IPv6 overlay addressing documentation (#667)

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```.