Utilities/Release: Add docker spec for STGZ package on Linux

The spec added by commit facc240a45 (Utilities/Release: Add docker specs
to build and test Linux binaries, 2019-08-23) generates only the TGZ
package, but the old `Utilities/Release/linux64_release.cmake` script
also generated a STGZ package.

Utilities/Release/README.rst

@@ -34,7 +34,7 @@ may use to produce Linux binaries for CMake:
         -f cmake-src/Utilities/Release/linux/$arch/Dockerfile cmake-src
     $ docker container create --name cmake-build cmake:build
     $ docker cp cmake-build:/out .
-    $ ls out/cmake-*-Linux-$arch.tar.gz
+    $ ls out/cmake-*-Linux-$arch.*
 
 * ``linux/<arch>/test/Dockerfile``:
   Produces a base image with a test environment for packaged CMake binaries.

Utilities/Release/linux/x86_64/Dockerfile

@@ -29,7 +29,8 @@ RUN : \
         bin/ctest --output-on-failure -j 8 -R '^(CMake\.|CMakeLib\.|CMakeServerLib\.|RunCMake\.ctest_memcheck)'; \
     fi \
  && bin/cpack -G TGZ \
+ && bin/cpack -G STGZ \
  && set +x \
  && mkdir /out \
- && mv cmake-*-Linux-x86_64.tar.gz /out \
+ && mv cmake-*-Linux-x86_64.* /out \
  && :