general purpose GitLab CI style check with prettier, plus local shell script to format with prettier. This ensures that the local formatting and the remote style check use the same version, and little effort is required on the local end

.gitlab-ci.yml

prettier_style_check:
  image: node:latest
  stage: test
  script:
    - npm install -g prettier
    - prettier --version
    - prettier . --check || { echo "did you run \`format.sh\`?"; exit 1; }

format.sh

#!/bin/bash
set -e

CONTAINER_NAME="gitlab-prettier-ci"

echo "I will build a docker container on your machine (about 1G), bind mount your current working directory, and format the files in that directory with \`prettier\`. If I have already built a container, I won't build another."
echo "If that's ok with you, press enter. Else, press ctrl+C."
read confirm_ok

if [ ! -z "$(docker images -q $CONTAINER_NAME)" ]; then
    docker run -v "$PWD:/app" $CONTAINER_NAME
    exit 0
fi

docker build --tag $CONTAINER_NAME --file - <<- DOCKERFILE
    FROM node:latest
    WORKDIR /app
    RUN npm install -g prettier
    CMD ["prettier", "/app", "--write"]
DOCKERFILE

docker run -v "$PWD:/app" $CONTAINER_NAME