feat: auto-recreate sandbox containers on config change

docs/gateway/configuration.md

@@ -1990,6 +1990,9 @@ cross-session isolation. Use `scope: "session"` for per-session isolation.
 Legacy: `perSession` is still supported (`true` → `scope: "session"`,
 `false` → `scope: "shared"`).
 
+`setupCommand` runs **once** after the container is created (inside the container via `sh -lc`).
+For package installs, ensure network egress, a writable root FS, and a root user.
+
 ```json5
 {
   agents: {

docs/gateway/sandboxing.md

@@ -116,6 +116,20 @@ Override with `agents.defaults.sandbox.docker.network`.
 Docker installs and the containerized gateway live here:
 [Docker](/install/docker)
 
+## setupCommand (one-time container setup)
+`setupCommand` runs **once** after the sandbox container is created (not on every run).
+It executes inside the container via `sh -lc`.
+
+Paths:
+- Global: `agents.defaults.sandbox.docker.setupCommand`
+- Per-agent: `agents.list[].sandbox.docker.setupCommand`
+
+
+Common pitfalls:
+- Default `docker.network` is `"none"` (no egress), so package installs will fail.
+- `readOnlyRoot: true` prevents writes; set `readOnlyRoot: false` or bake a custom image.
+- `user` must be root for package installs (omit `user` or set `user: "0:0"`).
+
 ## Tool policy + escape hatches
 Tool allow/deny policies still apply before sandbox rules. If a tool is denied
 globally or per-agent, sandboxing doesn’t bring it back.