Two instances of WordPress result randomly with status 301 - wordpress

Question

I have exactly same containers containing WordPress app on docker swarm. They are both on the same network.
The problem is that after WordPress installation when I send GET to first app curl -I $IP1 randomly I get response 301 with link to IP2...
The first question is - how the heck does one instance know about another? How can I debug what is the reason of such redirection?
And the main question is - how to fix it.
wordpress.yml
version: '3.5'
services:
db:
image: mysql:5.7
networks:
- proxynet
volumes:
- db-data:/var/lib/mysql
restart: always
environment:
MYSQL_ROOT_PASSWORD: changeme
MYSQL_DATABASE: wordpress
MYSQL_USER: wp
MYSQL_PASSWORD: changemetoo
deploy:
placement:
constraints:
- node.role == manager
word:
depends_on:
- db
image: wordpress
networks:
- proxynet
volumes:
- wp-content:/var/www/html
environment:
WORDPRESS_DB_HOST: db:3306
WORDPRESS_DB_NAME: wordpress
WORDPRESS_DB_USER: wp
WORDPRESS_DB_PASSWORD: changemetoo
volumes:
db-data:
wp-content:
network.yml:
networks:
proxynet:
name: proxynet
part of wordpress log:
10.0.0.4 - - [25/Oct/2018:14:49:54 +0000] "HEAD / HTTP/1.1" 301 209 "-" "curl/7.35.0"
10.0.0.4 - - [25/Oct/2018:14:49:56 +0000] "HEAD / HTTP/1.1" 301 209 "-" "curl/7.35.0"
10.0.0.4 - - [25/Oct/2018:14:49:58 +0000] "HEAD / HTTP/1.1" 301 209 "-" "curl/7.35.0"
10.0.0.4 - - [25/Oct/2018:14:50:00 +0000] "HEAD / HTTP/1.1" 200 221 "-" "curl/7.35.0"
10.0.0.4 - - [25/Oct/2018:14:50:02 +0000] "HEAD / HTTP/1.1" 301 209 "-" "curl/7.35.0"
10.0.0.4 - - [25/Oct/2018:14:50:04 +0000] "HEAD / HTTP/1.1" 301 209 "-" "curl/7.35.0"

Answer 1

I thought there was a bug in docker and I have created issue there.
But it was not.
If anyone would have a problem similar to this one, there is splendid answer from #thaJeztah on github