[Docs] Pin to specific version of Python in the example

Python default (`latest`) Docker image recently switched to Debian 12
which uses PEP668 (marking Python base environments as "externally
managed"). This change is not yet supported by GSC, so let's pin the
Python example to a supported version.

Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>

Documentation/index.rst

@@ -469,29 +469,30 @@ This example assumes that all prerequisites are installed and configured.
 
       openssl genrsa -3 -out enclave-key.pem 3072
 
-#. Pull public Python image from Dockerhub:
+#. Pull public Python image from Dockerhub (we pin to the Debian Bullseye
+   version):
 
    .. code-block:: sh
 
-      docker pull python
+      docker pull python:bullseye
 
 #. Graminize the Python image using :command:`gsc build`:
 
    .. code-block:: sh
 
-      ./gsc build --insecure-args python test/generic.manifest
+      ./gsc build --insecure-args python:bullseye test/generic.manifest
 
 #. Sign the graminized Docker image using :command:`gsc sign-image`:
 
    .. code-block:: sh
 
-      ./gsc sign-image python enclave-key.pem
+      ./gsc sign-image python:bullseye enclave-key.pem
 
 #. Retrieve SGX-related information from graminized image using :command:`gsc info-image`:
 
    .. code-block:: sh
 
-      ./gsc info-image gsc-python
+      ./gsc info-image gsc-python:bullseye
 
 #. Test the graminized Docker image (change ``--device=/dev/sgx_enclave`` to
    your version of the Intel SGX driver if needed):
@@ -500,7 +501,7 @@ This example assumes that all prerequisites are installed and configured.
 
       docker run --device=/dev/sgx_enclave \
          -v /var/run/aesmd/aesm.socket:/var/run/aesmd/aesm.socket \
-         gsc-python -c 'print("HelloWorld!")'
+         gsc-python:bullseye -c 'print("HelloWorld!")'
 
 #. You can also start a Bash interactive session in the graminized Docker
    image (useful for debugging):
@@ -509,7 +510,7 @@ This example assumes that all prerequisites are installed and configured.
 
       docker run --device=/dev/sgx_enclave \
          -v /var/run/aesmd/aesm.socket:/var/run/aesmd/aesm.socket \
-         -it --entrypoint /bin/bash gsc-python
+         -it --entrypoint /bin/bash gsc-python:bullseye
 
 Limitations
 ===========