docs: add Docker/sandbox tip to installation guide

Brief note under Quick Install covering plugin-specific container
details: install command, config mount, companion skip, auto-update
cache caveat. Closed #599 with upstream pointer.

docs/installation.md

@@ -27,6 +27,8 @@ Or use non-interactive mode:
 bunx oh-my-opencode-slim@latest install --no-tui --skills=yes --background-subagents=yes
 ```
 
+> **Running in Docker or a sandbox?** Install inside the container with the same command, and mount `~/.config/opencode` for persistence. Pass `--companion=no` to skip the desktop companion (it requires a display server). Auto-updates write to `~/.cache/opencode/packages/`, which is separate from config — mount that too if you want updates to survive container restarts.
+
 ### Configuration Options
 
 The installer supports the following options: