Renamed some docker files to Dockerfile

This makes it simpler to manage by using the default name. It also helps
my editor to recognise the file and highlight it properly.

docker/3_buildwheels/build.sh

@@ -22,7 +22,7 @@ fi
 echo "Wheelhouse is $WHEELHOUSE"
 mkdir -p "$WHEELHOUSE"
 
-docker build -t pillar_wheelbuilder -f build.docker .
+docker build -t pillar_wheelbuilder .
 
 GID=$(id -g)
 docker run --rm -i \