rename LLAMASTACK_PORT to LLAMA_STACK_PORT for consistency with other env vars (#744)

# What does this PR do?

Rename environment var for consistency

## Test Plan

No regressions

## Sources

## Before submitting

- [X] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [X] Ran pre-commit to handle lint / formatting issues.
- [X] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
Pull Request section?
- [X] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

---------

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>
Co-authored-by: Yuan Tang <terrytangyuan@gmail.com>

llama_stack/distribution/start_container.sh

@@ -90,6 +90,6 @@ $DOCKER_BINARY run $DOCKER_OPTS -it \
   $env_vars \
   -v "$yaml_config:/app/config.yaml" \
   $mounts \
-  --env LLAMASTACK_PORT=$port \
+  --env LLAMA_STACK_PORT=$port \
   --entrypoint='["python", "-m", "llama_stack.distribution.server.server", "--yaml-config", "/app/config.yaml"]' \
   $docker_image:$version_tag