Reworked the Dockerfile a bit.

Now requirements.txt is installed first and includes
setuptools >= 78.1.1, so removed explicit setuptools install
from pip install gpg command line. Added --no-build-isolation to pip 
command to install gpg. Also added --no-cache-dir to gpg install
get newest version.

fix in changeset:   8825:95448795dbd4

Thanks Chris and Bern for the fix.

Issue 2551430 opened because the license settings in gpg's
build config will cause build issues starting feb 2027.