Syntax error when adding SSH key in GitLab CI

I have tried many times, the following .gitlab-ci.yml should work properly.

    image: gitlab/dind:latest

variables:
  COMPOSE: docker-compose

before_script:
  # Install ssh-agent if not already installed, it is required by Docker.
  # (change apt-get to yum if you use a CentOS-based image)
  - 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'

  # Run ssh-agent (inside the build environment)
  - eval $(ssh-agent -s)

  # Add the SSH key stored in SSH_PRIVATE_KEY variable to the agent store
  - ssh-add <(echo "$SSH_PRIVATE_KEY")

  # For Docker builds disable host key checking. Be aware that by adding that
  # you are suspectible to man-in-the-middle attacks.
  # WARNING: Use this only with the Docker executor, if you use it with shell
  # you will overwrite your user's SSH config.
  - mkdir -p ~/.ssh

  - ssh-keyscan -H 'gitlab.com' >> ~/.ssh/known_hosts
  - ssh-keyscan gitlab.com | sort -u - ~/.ssh/known_hosts -o ~/.ssh/known_hosts

  - '[[ -f /.dockerinit ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'

stages:
  - build
  - test
  - deploy

# Add a job called 'build' -> to run your builds
# TODO: Build the image in remote docker registry 
build-dev:
  stage: build 
  script:
    - sh scripts/install-dev.sh
    - $COMPOSE build
  only:
    - dev 

build-master:
  stage: build 
  script:
    - sh scripts/install.sh
    - $COMPOSE build
  only:
    - master 

I have also opened an issue here.

To make it work I had to replace:

ssh-add <(echo "$SSH_PRIVATE_KEY")

with:

printenv SSH_PRIVATE_KEY | ssh-add -