Fix Docker Build CI by correcting backend requirements path in Dockerfile

[Copilot]

The Docker Build GitHub Actions job failed because the builder stage tried to copy requirements.txt from the repository root, but the backend dependency file is located under rag-system/. This PR updates the Docker build input path so image builds can resolve Python dependencies correctly.

• Root cause

  • Builder stage referenced a non-existent file path:
    • COPY requirements.txt .

• Change

  • Updated the builder stage to copy the actual backend requirements file:
    • COPY rag-system/requirements.txt .

• Impact

  • Aligns Dockerfile path resolution with repository layout.
  • Unblocks the CI Docker Build job from failing at dependency copy/install stage.

# before
COPY requirements.txt .
RUN pip install --no-cache-dir --prefix=/install -r requirements.txt

# after
COPY rag-system/requirements.txt .
RUN pip install --no-cache-dir --prefix=/install -r requirements.txt

[Aathi-27]

verified

[Aathi-27]

approved