you have to run docker network inspect bridge

the result will be :

[
    {
        "Name": "bridge",
        "Id": "a82774b4cc857b934ca5d3bba8f20a077aef3c117b9a390455466fb8785aea0f",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                    "Gateway": "172.17.0.1"
                }
            ]
        },
        "Internal": false,
        "Containers": {
            "64e285ed8b6a287c693df8244f4cf45c6e724e74e979734b823f14f19cb2a74b": {
                "Name": "node-bb-redis",
                "EndpointID": "869f51cfefb3d1d59b5f0cc881c16e6a127b423345704c7f17bc3f10c5061922",
                "MacAddress": "02:42:ac:11:00:02",
                "IPv4Address": "172.17.0.2/16",
                "IPv6Address": ""
            },
            "7fd7885220b189427121102a9cf34e7b2259b5d1038b3f6a522f24e1e3ebd25b": {
                "Name": "node-bb",
                "EndpointID": "9d971bdd8675714ba96385c3832cf08a0ed6630cc40e8ac08d3666c42f0b9133",
                "MacAddress": "02:42:ac:11:00:05",
                "IPv4Address": "172.17.0.5/16",
                "IPv6Address": ""
            },
            "8391d3e8f44f064e885e2cf754a59a7cc00d24ca5c2db13b4add0152308eb843": {
                "Name": "nginx-letsencrypt",
                "EndpointID": "0416947f3ff616fb063b17363ef4f5d282a0549bc15582a842f97155b9ede5c2",
                "MacAddress": "02:42:ac:11:00:04",
                "IPv4Address": "172.17.0.4/16",
                "IPv6Address": ""
            },
            "a06b204eb61be007c093488aa1f3491407b784c48c4b3a069afcc967892b1522": {
                "Name": "nginx-proxy",
                "EndpointID": "6de08ad911df8e80b04f4600909b76c70d3dd99f26a586cc444bf5a2ec97e2ef",
                "MacAddress": "02:42:ac:11:00:03",
                "IPv4Address": "172.17.0.3/16",
                "IPv6Address": ""
            }
        },
        "Options": {
            "com.docker.network.bridge.default_bridge": "true",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1500"
        },
        "Labels": {}
    }
]

Find your redis IP and put it in your nodeBB config (in my case Redis IP is 172.17.0.2)