Permission denied (publickey,password). Gitlab CI/CD - docker

Question

My Gilab CI script falls and exists with this error
Permission denied, please try again.
Permission denied, please try again.
$SSH_USER#$IPADDRESS: Permission denied (publickey,password).
This is my CI script:
image: alpine
before_script:
- echo "Before script"
- apk add --no-cache rsync openssh openssh-client
- mkdir -p ~/.ssh
- eval $(ssh-agent -s)
- echo "$SSH_KNOWN_HOSTS" > ~/.ssh/known_hosts
- chmod 644 ~/.ssh/known_hosts
- echo "${SSH_PRIVATE_KEY}" | tr -d '\r' | ssh-add - > /dev/null
- ssh -o 'StrictHostKeyChecking no' $SSH_USER#$IPADDRESS
- cd /var/www/preview.hidden.nl/test
building:
stage: build
script:
- git reset --hard
- git pull origin develop
- composer install
- cp .env.example .env
- php artisan key:generate
- php artisan migrate --seed
- php artisan cache:clear
- php artisan config:clear
- php artisan storage:link
- sudo chown -R deployer:www-data /var/www/preview.hidden.nl/test/
- find /var/www/preview.hidden.nl/test -type f -exec chmod 664 {} \;
- find /var/www/preview.hidden.nl/test -type d -exec chmod 775 {} \;
- chgrp -R www-data storage bootstrap/cache
- chmod -R ug+rwx storage bootstrap/cache
My setup is as follows.
Gitlab server > Gitlab.com
Gitlab runner > Hetzner server
Laravel Project > Same Hetzner server
I generated a new SSH-key (without password) pair for this runner, named gitlab & gitlab.pub. I added the content of "gitlab.pub" to the $KNOWN_HOST variable. I added the content of "gitlab" to the $SSH_PRIVATE_KEY variable.
The problem is, I don't really know what's going on. What I think is happening is the following. The GitLab Ci job is its own separate container. A container can't just ssh to a remote server. So the private key of my Hetzner server needs to be known to the docker container ( task: - echo "$SSH_KNOWN_HOSTS" > ~/.ssh/known_hosts ).
Because the key is then known, the docker container should not ask for a password. Yet it prompts for a password and also returns that the password is incorrect. I also have my own private key pair, besides the GitLab key pair, not sure if that causes the issue; but removing my own key there would block access to my server, so I did not test removing that.
Could someone help me in this manner? I've been working on this for two weeks and I can't even deploy a simple hello-world.txt file yet! :-)