Switch from id --group id -g

This makes the script compatible with macOS.

docker/2_buildpy/build.sh

@@ -18,7 +18,7 @@ echo "Python will be built to $PYTHONTARGET"
 docker build -t pillar_build -f buildpy.docker .
 
 # Use the docker image to build Python 3.6 and mod-wsgi
-GID=$(id --group)
+GID=$(id -g)
 docker run --rm -i \
     -v "$PYTHONTARGET:/opt/python" \
     pillar_build <<EOT

docker/3_buildwheels/build.sh

@@ -24,7 +24,7 @@ mkdir -p "$WHEELHOUSE"
 
 docker build -t pillar_wheelbuilder -f build.docker .
 
-GID=$(id --group)
+GID=$(id -g)
 docker run --rm -i \
     -v "$WHEELHOUSE:/data/wheelhouse" \
     -v "$TOPDEVDIR:/data/topdev" \