How to use Dockerfile in Gitlab CI

Using gitlab-ci for my node/react app, I'm trying to use phusion/passenger-nodejs as the base docker image

I can specify this easily in .gitlab-ci.yml:

image: phusion/passenger-nodejs:latest

variables:
  HOME: /root

cache:
  paths:
  - node_modules/

stages:
  - build
  - test
  - deploy

set_environment:
  stage: build
  script:
    - npm install
  tags:
  - docker

test_node:
  stage: test
  script:
    - npm install
    - npm test
  tags:
    - docker

However, Phusion Passenger expects you to make configuration changes, e.g. python support, using their special init process, etc. in the Dockerfile.

#FROM phusion/passenger-ruby24:<VERSION>
#FROM phusion/passenger-jruby91:<VERSION>
FROM phusion/passenger-nodejs:<VERSION>
#FROM phusion/passenger-customizable:<VERSION>

# Set correct environment variables.
ENV HOME /root

# Use baseimage-docker's init process.
CMD ["/sbin/my_init"]

# If you're using the 'customizable' variant, you need to explicitly opt-in
# for features. 
#
# N.B. these images are based on https://github.com/phusion/baseimage-docker, 
# so anything it provides is also automatically on board in the images below 
# (e.g. older versions of Ruby, Node, Python).  
# 
# Uncomment the features you want:
#
#   Ruby support
#RUN /pd_build/ruby-2.0.*.sh
#RUN /pd_build/ruby-2.1.*.sh
#RUN /pd_build/ruby-2.2.*.sh
#RUN /pd_build/ruby-2.3.*.sh
#RUN /pd_build/ruby-2.4.*.sh
#RUN /pd_build/jruby-9.1.*.sh
#   Python support.
RUN /pd_build/python.sh
#   Node.js and Meteor standalone support.
#   (not needed if you already have the above Ruby support)
RUN /pd_build/nodejs.sh

# ...put your own build instructions here...

# Clean up APT when done.
RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

Is there a way to use a Dockerfile with gitlab-ci? Is there a good work around other than apt-get install and adding shell scripts?

Yes, create a second Gitlab repository where you place your Dockerfile in. There you add a gitlab-ci.yml file with a script command that builds you modified image and push it to your private registry or the Gitlab embedded Docker registry, eg:

script: docker build . -t http://myregistry:5000/mymodified image docker push http://myregistry:5000/mymodified

Inside your other Gitlab repository, change the image: line accordingly:

image: http://myregistry:5000/mymodified

Information on the Gitlab embedded Docker registry can be found here -> here