Services in docker compose not recognizing each other - docker

Question

I am trying to make a Confluence container communicate with a mysql container. They are both services that are launched in my docker-compose.yml
version: '2'
services:
db:
image: mysql:5.7
networks:
- confluencenet
volumes:
- ./mycustom.cnf:/etc/mysql/conf.d/my.cnf
restart: always
command: [mysqld, --character-set-server=utf8, --collation-server=utf8_bin, --default-storage-engine=INNODB, --max_allowed_packet=256M, --innodb_log_file_size=2GB, --transaction-isolation=READ-COMMITTED, --binlog_format=row]
environment:
MYSQL_DATABASE: bachelorarbeit_database
# So you don't have to use root, but you can if you like
MYSQL_USER: horizon
# You can use whatever password you like
MYSQL_PASSWORD: 1P#ssw0rt123
# Password for root access
MYSQL_ROOT_PASSWORD: P#ssw0rt
MYSQL_ROOT_USER: root
ports:
# <Port exposed> : < MySQL Port running inside container>
- '3306:3306'
confluence:
image: techgeeks/confluence-mysql-server
networks:
- confluencenet
build:
context: ./
args:
CONFLUENCE_VERSION: 7.4.0
container_name: confluence
volumes:
- confluencedata:/var/atlassian/confluence
ports:
- '8090:8090'
- '8091:8091'
volumes:
confluencedata:
external: false
postgresqldata:
external: false
networks:
confluencenet: {}
From my understanding, two docker services need to share the same network if they have to communicate with each other. They work seperately, I can connect to each from outside. However, the confluence instance cannot connect to the db service.
SQLState - 08S01
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
What did I do wrong putting the two in the same docker network?