Replace sphinx pypi package with os py3 package

[bobidle]

I tried to build the Docker image via the provided Dockerfile and the build failed with the following error message

error: externally-managed-environment

× This environment is externally managed
╰─> 
    The system-wide python installation should be maintained using the system
    package manager (apk) only.
    
    If the package in question is not packaged already (and hence installable via
    "apk add py3-somepackage"), please consider installing it inside a virtual
    environment, e.g.:
    
    python3 -m venv /path/to/venv
    . /path/to/venv/bin/activate
    pip install mypackage
    
    To exit the virtual environment, run:
    
    deactivate
    
    The virtual environment is not deleted, and can be re-entered by re-sourcing
    the activate file.
    
    To automatically manage virtual environments, consider using pipx (from the
    pipx package).

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

Because of that, i replaced the installation of the pypi sphinx package with the os package py3-sphinx. And also used a tag for the Alpine Linux Docker image to make the build more reliable.

That worked for me. I also noticed some label issues #1064 resulting in a build error.

[rgerhards]

Thx for this PR and the others. I noticed them, but am currently busy. This sounds like a good and useful project!