Add env var gotcha to start command docs

Environment variables passed at Sandbox.create() time are not available
to the start command process since it already ran during template build.
This is a common source of confusion for users coming from Docker.

Author: beran-t

docs/template/start-ready-command.mdx

@@ -12,6 +12,8 @@ This is how you get servers, seeded databases, or any long-running process avail
 
 <Note title="The start command does not run on sandbox creation">
   The start command runs **once during template build** and is captured in a snapshot. It does not re-execute each time you create a sandbox. If you need to run a command every time a sandbox is created, use `sandbox.commands.run()` after creating the sandbox instead.
+
+  This also means that [environment variables passed to `Sandbox.create()`](/docs/sandbox/environment-variables#1-global-environment-variables) are **not available** to the start command process — it already ran during the build. If your start command needs environment variables, set them in the template definition using `setEnvs()` / `set_envs()`.
 </Note>
 
 You can see the full build process [here](/docs/template/how-it-works).