[Python] Docker build fails on pip install with multiple private PyPI indexes using...

I'm trying to build a Docker image for my Python/FastAPI project. My requirements.txt includes packages from both public PyPI and two private GitLab PyPI repositories.

Dockerfile excerpt:

FROM *****:*-slim-bullseye

ARG FUNCTION_DIR
WORKDIR ${FUNCTION_DIR}

COPY --from=build-image / /

ARG PIP_EXTRA_INDEX_URL
ENV PIP_EXTRA_INDEX_URL=${PIP_EXTRA_INDEX_URL}
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt


Build command:

docker build --progress=plain --build-arg PIP_EXTRA_INDEX_URL="https://gitlab.*********.com/apie/SE/projects/145/packages/pypi/simple/ https://username:token@gitlab.**************.com/apie/SE/projects/888/packages/pypi/simple/" -t fast_api .


requirements.txt:

fastapi[standard]==0.115.12
mangum==0.19.0
uvicorn==0.34.0
MTPTS[full]>=1.99.44
TSSA>=1.99.1


Problem:
The build fails at the pip install step with a generic error:

ERROR: failed to solve: process "/bin/sh -c pip install -r requirements.txt" did not complete successfully: exit code: 1
1440 /usr/local/bin/dockerd --containerd /run/containerd/containerd.sock --pidfile /run/desktop/docker.pid --swarm-default-advertise-addr=eth0 --host-gateway-ip 192.***.**.***


I have confirmed that:

• 
  The PIP_EXTRA_INDEX_URL is quoted in the build command.
  
  
• 
  The credentials are correct.
  
  
• 
  The Dockerfile uses ARG and ENV for PIP_EXTRA_INDEX_URL.
  

Questions:

1. 
   Is my usage of PIP_EXTRA_INDEX_URL with multiple indexes correct for pip in Docker?
   
   
2. 
   Is there a better way to pass multiple private indexes with authentication to pip in a Docker build?
   
   
3. 
   How can I debug what pip is actually doing inside the container to see which index it is failing on?
   

Any help or suggestions are appreciated!

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