[devbox] Update README: clarify docker requirement (#2631)

TSIA

Author: loreto

README.md

@@ -52,7 +52,7 @@ In addition to installing Devbox itself, you will need to install `nix` and `doc
 
 1. Install [Nix Package Manager](https://nixos.org/download.html). (Don't worry, you don't need to learn Nix.)
 
-2. Install [Docker Engine](https://docs.docker.com/engine/install/) or [Docker Desktop](https://www.docker.com/get-started/).
+2. Install [Docker Engine](https://docs.docker.com/engine/install/) or [Docker Desktop](https://www.docker.com/get-started/). Note that docker is only needed if you want to create containers – the shell functionality works without it.
 
 3. Install Devbox: