From 3740a8bcb4fc2e82f47bec73ece8a80be31a667e Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Sun, 24 Jan 2021 18:23:29 +0100
Subject: [PATCH] docs/conf: add env var substitution docs

---
 docs/conf/writing.md | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/docs/conf/writing.md b/docs/conf/writing.md
index 05babd66d5..057675c98e 100644
--- a/docs/conf/writing.md
+++ b/docs/conf/writing.md
@@ -161,3 +161,19 @@ Example `my-secrets.json` file:
   }
 }
 ```
+
+## Environment Variable Substitution
+
+Configuration files support environment variable substitution via a `${MY_VAR}`
+syntax. For example:
+
+```yaml
+app:
+  baseUrl: https://${HOST}
+```
+
+Note that all environment variables must be available, or the entire
+configuration value will evaluate to `undefined`.
+
+The substitution syntax can be escaped using `$${...}`, which will be resolved
+as `${...}`.