feat: drop github.com/a8m/envsubst in favor of gookit/config (#9028)

docs/ocis/config.md

@@ -44,7 +44,7 @@ Let's explore with examples this approach.
 - docker images: `/etc/ocis/`
 - binary releases: `$HOME/.ocis/config/`
 
-followed by the `<extension name>.yaml`, eg `proxy.yaml` for the extension configuration. You also can put an `ocis.yaml` config file to the expected loading location to use a single config file.
+followed by the `<extension name>.yaml`, e.g. `proxy.yaml` for the extension configuration. You also can put an `ocis.yaml` config file to the expected loading location to use a single config file.
 
 You can set another directory as config path in the environment variable `OCIS_CONFIG_DIR`. It will then pick the same file names, but from the folder you configured.
 
@@ -99,6 +99,21 @@ log:
   level: info
 ```
 
+#### Substitute ENV variables in configuration files
+
+Environment variables can be used in the configurations files and will be replaced by oCIS when loading these.
+Default values can be specified after a `|` character - see below.
+
+```yaml
+proxy:
+  http:
+    addr: ${PROXY_HTTP_ADDR|localhost:4321}
+log:
+  pretty: true
+  color: true
+  level: info
+```
+
 ### Workflows
 
 Since one can run an extension using the runtime (supervised) or not (unsupervised), we ensure correct behavior in both modes, expecting the same outputs.