rostopic list show the topics from the opposite docker, but rostopic echo cannot read the content in a topic from the opposite docker

edit

I'm currently using ROS melodic locally because of some legacy codes.

However, I should upgrade this version soon.

Before upgrading the version, to prevent the environment from messing up, I should run ROS melodic in each docker on two different PC and check the communication between the ends.

[melodic(docker) - RaspberryPI <=> RemotePC - melodic(docker)]

My environment is composed of 2 PC like below

Environment : 2 PC(Raspberry PI / Remote PC)
1. Raspberry PI : raspbian
2. Remote PC : Ubuntu 18.04 => (it is gonna be upgraded to 20.04)

what I did

-- Two dockers network sets host to use each local IP(using same internal network)
-- replaced the ROS_MASTER_URI and ROS_HOSTNAME with http://Remote_PC_IP:11311 and Remote_PC_IP respectively
(on ubuntu18.04 in docker at ubuntu)
-- replaced the ROS_MASTER_URI and ROS_HOSTNAME with http://Remote_PC_IP:11311 and RaspberryPI_IP respectively
(on ubuntu18.04 in docker at raspberry PI)
-- roscore firstly, and rostopic pub msg std_msgs/String Hello(on ubuntu18.04 in docker at ubuntu)
-- check the topic list(on ubuntu18.04 in docker at raspberry PI)

I can see the topic list well, but I cannot check the content. Please let me know what I miss, if you know it.

Thank you, in advance, sincerely.