Missing prerequisites with docker buildx

[piotrm-nvidia]

JIRA: DLIS-7832

There are no prerequisites for repo and example. The build.sh needs not only docker installed but also docker-buildx-plugin extension for docker to work. Let's describe prerequisites in separate readme section and add these.

The user also needs to create account in NGC and do docker login nvcr.io

The build script fails without this step:

 > [internal] load metadata for nvcr.io/nvidia/tritonserver:24.12-vllm-python-py3:
------
Dockerfile:20
--------------------
  18 |     ARG VLLM_WHEEL
  19 |     
  20 | >>> FROM ${BASE_IMAGE}:${BASE_IMAGE_TAG} AS triton-distributed
  21 |     
  22 |     # TODO: non root user by default
--------------------
ERROR: failed to solve: nvcr.io/nvidia/tritonserver:24.12-vllm-python-py3: failed to authorize: failed to fetch oauth token: unexpected status from GET request to https://nvcr.io/proxy_auth?scope=repository%3Anvidia%2Ftritonserver%3Apull: 401