image: docker:latest

stages:
    ...

build:
  stage: build
  script:
  - chmod +x start.sh
  - docker build -t MY_IMAGE:TAG .

...

FROM php:7.2-fpm-alpine3.7

RUN ... (the big RUN chunk from the nginx Dockerfile linked below)

RUN ... (my custom stuff)

EXPOSE 80

STOPSIGNAL SIGTERM

COPY nginx.conf /etc/nginx/nginx.conf
COPY start.sh /usr/local/bin/start.sh

CMD ["start.sh"]

php-fpm -D
nginx -g 'daemon off;'