[Docs] document new environment variable settings

docs/finn/getting_started.rst

@@ -131,6 +131,9 @@ These are summarized below:
 * (optional) ``PYNQ_TARGET_DIR`` (or ``ALVEO_TARGET_DIR``) specifies the target dir on the PYNQ board / Alveo host for the test suite
 * (optional) ``FINN_HOST_BUILD_DIR`` specifies which directory on the host will be used as the build directory. Defaults to ``/tmp/finn_dev_<username>``
 * (optional) ``IMAGENET_VAL_PATH`` specifies the path to the ImageNet validation directory for tests.
+* (optional) ``FINN_DOCKER_PREBUILT`` (default 0) if set to 1 then skip Docker image building and use the image tagged with ``FINN_DOCKER_TAG``.
+* (optional) ``FINN_DOCKER_RUN_AS_ROOT`` (default 0) if set to 1 then run Docker container as root, default is the current user.
+* (optional) ``FINN_DOCKER_GPU`` (autodetected) if not 0 then expose all Nvidia GPUs to Docker container for accelerated DNN training.
 
 Supported Hardware
 ===================