Update FAQ: clarify about hashed pass and compose file (coder#6671)

Author: ckesc

docs/FAQ.md

@@ -356,6 +356,12 @@ hashed-password: "$argon2i$v=19$m=4096,t=3,p=1$wST5QhBgk2lu1ih4DMuxvg$LS1alrVdIW
 
 The `hashed-password` field takes precedence over `password`.
 
+If you're using Docker Compose file, in order to make this work, you need to change all the single $ to $$. For example:
+
+```yaml
+- HASHED_PASSWORD=$$argon2i$$v=19$$m=4096,t=3,p=1$$wST5QhBgk2lu1ih4DMuxvg$$LS1alrVdIWtvZHwnzCM1DUGg+5DTO3Dt1d5v9XtLws4
+```
+
 ## Is multi-tenancy possible?
 
 If you want to run multiple code-servers on shared infrastructure, we recommend