add support for ${env.FOO_BAR} placeholders in run.yaml files (#439)

# What does this PR do?

We'd like our docker steps to require _ZERO EDITS_ to a YAML file in
order to get going. This is often not possible because depending on the
provider, we do need some configuration input from the user. Environment
variables are the best way to obtain this information.

This PR allows our run.yaml to contain `${env.FOO_BAR}` placeholders
which can be replaced using `docker run -e FOO_BAR=baz` (and similar
`docker compose` equivalent).

## Test Plan

For remote-vllm, example `run.yaml` snippet looks like this:
```yaml
providers:
  inference:
  # serves main inference model
  - provider_id: vllm-0
    provider_type: remote::vllm
    config:
      # NOTE: replace with "localhost" if you are running in "host" network mode
      url: ${env.LLAMA_INFERENCE_VLLM_URL:http://host.docker.internal:5100/v1}
      max_tokens: ${env.MAX_TOKENS:4096}
      api_token: fake
  # serves safety llama_guard model
  - provider_id: vllm-1
    provider_type: remote::vllm
    config:
      # NOTE: replace with "localhost" if you are running in "host" network mode
      url: ${env.LLAMA_SAFETY_VLLM_URL:http://host.docker.internal:5101/v1}
      max_tokens: ${env.MAX_TOKENS:4096}
      api_token: fake
```

`compose.yaml` snippet looks like this:
```yaml
llamastack:
    depends_on:
    - vllm-0
    - vllm-1
      # image: llamastack/distribution-remote-vllm
    image: llamastack/distribution-remote-vllm:test-0.0.52rc3
    volumes:
      - ~/.llama:/root/.llama
      - ~/local/llama-stack/distributions/remote-vllm/run.yaml:/root/llamastack-run-remote-vllm.yaml
    # network_mode: "host"
    environment:
      - LLAMA_INFERENCE_VLLM_URL=${LLAMA_INFERENCE_VLLM_URL:-http://host.docker.internal:5100/v1}
      - LLAMA_INFERENCE_MODEL=${LLAMA_INFERENCE_MODEL:-Llama3.1-8B-Instruct}
      - MAX_TOKENS=${MAX_TOKENS:-4096}
      - SQLITE_STORE_DIR=${SQLITE_STORE_DIR:-$HOME/.llama/distributions/remote-vllm}
      - LLAMA_SAFETY_VLLM_URL=${LLAMA_SAFETY_VLLM_URL:-http://host.docker.internal:5101/v1}
      - LLAMA_SAFETY_MODEL=${LLAMA_SAFETY_MODEL:-Llama-Guard-3-1B}
```

llama_stack/distribution/server/server.py

@@ -8,6 +8,8 @@ import asyncio
 import functools
 import inspect
 import json
+import os
+import re
 import signal
 import sys
 import traceback
@@ -258,13 +260,66 @@ def create_dynamic_typed_route(func: Any, method: str):
     return endpoint
 
 
+class EnvVarError(Exception):
+    def __init__(self, var_name: str, path: str = ""):
+        self.var_name = var_name
+        self.path = path
+        super().__init__(
+            f"Environment variable '{var_name}' not set or empty{f' at {path}' if path else ''}"
+        )
+
+
+def replace_env_vars(config: Any, path: str = "") -> Any:
+    if isinstance(config, dict):
+        result = {}
+        for k, v in config.items():
+            try:
+                result[k] = replace_env_vars(v, f"{path}.{k}" if path else k)
+            except EnvVarError as e:
+                raise EnvVarError(e.var_name, e.path) from None
+        return result
+
+    elif isinstance(config, list):
+        result = []
+        for i, v in enumerate(config):
+            try:
+                result.append(replace_env_vars(v, f"{path}[{i}]"))
+            except EnvVarError as e:
+                raise EnvVarError(e.var_name, e.path) from None
+        return result
+
+    elif isinstance(config, str):
+        pattern = r"\${env\.([A-Z0-9_]+)(?::([^}]*))?}"
+
+        def get_env_var(match):
+            env_var = match.group(1)
+            default_val = match.group(2)
+
+            value = os.environ.get(env_var)
+            if not value:
+                if default_val is None:
+                    raise EnvVarError(env_var, path)
+                else:
+                    value = default_val
+
+            return value
+
+        try:
+            return re.sub(pattern, get_env_var, config)
+        except EnvVarError as e:
+            raise EnvVarError(e.var_name, e.path) from None
+
+    return config
+
+
 def main(
     yaml_config: str = "llamastack-run.yaml",
     port: int = 5000,
     disable_ipv6: bool = False,
 ):
     with open(yaml_config, "r") as fp:
-        config = StackRunConfig(**yaml.safe_load(fp))
+        config = replace_env_vars(yaml.safe_load(fp))
+        config = StackRunConfig(**config)
 
     app = FastAPI()