Make run yaml optional so dockers can start with just --env (#492)

When running with dockers, the idea is that users be able to work purely
with the `llama stack` CLI. They should not need to know about the
existence of any YAMLs unless they need to. This PR enables it.

The docker command now doesn't need to volume mount a yaml and can
simply be:
```bash
docker run -v ~/.llama/:/root/.llama \
  --env A=a --env B=b
```

## Test Plan

Check with conda first (no regressions):
```bash
LLAMA_STACK_DIR=. llama stack build --template ollama
llama stack run ollama --port 5001

# server starts up correctly
```

Check with docker
```bash
# build the docker
LLAMA_STACK_DIR=. llama stack build --template ollama --image-type docker

export INFERENCE_MODEL="meta-llama/Llama-3.2-3B-Instruct"

docker run -it  -p 5001:5001 \
  -v ~/.llama:/root/.llama \
  -v $PWD:/app/llama-stack-source \
  localhost/distribution-ollama:dev \
  --port 5001 \
  --env INFERENCE_MODEL=$INFERENCE_MODEL \
  --env OLLAMA_URL=http://host.docker.internal:11434
```

Note that volume mounting to `/app/llama-stack-source` is only needed
because we built the docker with uncommitted source code.

llama_stack/distribution/build_container.sh

@@ -122,7 +122,7 @@ add_to_docker <<EOF
 # This would be good in production but for debugging flexibility lets not add it right now
 # We need a more solid production ready entrypoint.sh anyway
 #
-ENTRYPOINT ["python", "-m", "llama_stack.distribution.server.server"]
+ENTRYPOINT ["python", "-m", "llama_stack.distribution.server.server", "--template", "$build_name"]
 
 EOF