website/docs: automated install: mention no file:// vars (#20043)

* Clarify environment variable usage for automated install

Add note about environment variable limitations in automated install guide.

Closes #11023

Signed-off-by: Tom Crasset <25140344+tcrasset@users.noreply.github.com>

* Apply suggestion from @dominic-r

Signed-off-by: Dominic R <dominic@sdko.org>

---------

Signed-off-by: Tom Crasset <25140344+tcrasset@users.noreply.github.com>
Signed-off-by: Dominic R <dominic@sdko.org>
Co-authored-by: Dominic R <dominic@sdko.org>

Author: tcrasset

website/docs/install-config/automated-install.mdx

@@ -4,6 +4,10 @@ title: Automated install
 
 To install authentik automatically (skipping the Out-of-box experience), you can use the following environment variables on the worker container:
 
+:::info
+These can't be defined using the file-based syntax (`file://`), so you can't pass them in as secrets in a Docker Compose installation.
+:::
+
 ### `AUTHENTIK_BOOTSTRAP_PASSWORD`
 
 Configure the default password for the `akadmin` user. Only read on the first startup. Can be used for any flow executor.