fix proxy race conditions

server/schema.sql

@@ -12,6 +12,7 @@ CREATE TABLE IF NOT EXISTS apps (
 CREATE TABLE IF NOT EXISTS containers (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     container_id TEXT NOT NULL,
+    head BOOLEAN NOT NULL,
     deployment_id INTEGER NOT NULL,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     FOREIGN KEY(deployment_id) REFERENCES deployments(id)