Docker PoC for batteries

eg4battery/bin/eg4-battery

@@ -100,9 +100,25 @@ class AppConfig:
 
 def load_config(path: Path) -> AppConfig:
     raw = yaml.safe_load(path.read_text())
+
+    # Allow env-var overrides for MQTT credentials. Useful for Docker
+    # deployments where secrets shouldn't live in the YAML, and for HA
+    # addons translating addon-options into env at runtime.
+    import os
+    mqtt_raw = dict(raw["mqtt"])
+    for key, env_var in (
+        ("host", "MQTT_HOST"),
+        ("port", "MQTT_PORT"),
+        ("username", "MQTT_USERNAME"),
+        ("password", "MQTT_PASSWORD"),
+    ):
+        v = os.environ.get(env_var)
+        if v is not None:
+            mqtt_raw[key] = int(v) if key == "port" else v
+
     return AppConfig(
         bus=BusConfig(**raw["bus"]),
-        mqtt=MQTTConfig(**raw["mqtt"]),
+        mqtt=MQTTConfig(**mqtt_raw),
         packs=[PackConfig(**p) for p in raw["packs"]],
         cell_count=raw.get("cell_count", 16),
     )