Add template build envs warning (#72)

Author: beran-t

docs/template/defining-template.mdx

@@ -243,6 +243,11 @@ template.git_clone("https://github.com/user/repo.git", "/app/repo", depth=1)
 
 ### Environment variables
 
+<Warning> 
+Environment variables set in template definition are only available during template build.
+[How to setup environment variables in sandbox?](/docs/sandbox/environment-variables)
+</Warning>
+
 Set environment variables in the template:
 
 <CodeGroup>