How to use Gitlab's Auto DevOps for multi-container application?

I have a multi-container application, with nginx as web server and reverse-proxy, and a simple 'Hello World' Streamlit app.
It is available on my Gitlab.

I am totally new to DevOps, and would therefore like to leverage Gitlab's Auto DevOps so as to make it easy.
By default Gitlab's Auto DevOps expects one Dockerfile only, and at the root of the project (source)
Surprisingly, I only found one ressource on my multi-container use case, that aimed to answer this issue : https://forum.gitlab.com/t/auto-build-for-multiple-docker-containers/46949
I followed the advice, and made only slights changes to the .gitlab-ci.yml for the path to my dockerfiles.

But then I have an issue with the Dockerfiles not recognizing the files in its folder : App's Dockerfile doesn't find the requirements.txt :

And Nginx's Dockerfile doesn't find the project.conf

It seems that the DOCKERFILE_PATH: src/nginx/Dockerfile variable gives only acess to the Dockerfile in itself, but doesn't understand this path as the location for the build.
How can I customize this .gitlab-ci.yml so that the build passes correctly ?
Thank you very much !

The reason the files are not being found is due to how docker's context works. Since you're running docker build from the root, your context will be within the root as opposed to from the path for your dockerfile. That means that your docker build command is trying to find /requirements.txt instead of src/app/requirements.txt. You can fix this relatively easily by just executing a cd to change to your /src/app directory before you run docker build, and removing the -f flag from your docker build (since you no longer need to specify the folder).

Since each job executes in an isolated container, you don't need to worry about CDing back to your build root, since your job never runs any other non-docker commands.