fix: temp migrate docs and add warning

See gotify/server#433

Author: jmattheis

docs/migrate-to-3.md

@@ -1,5 +1,10 @@
 # Migrate to 3.x
 
+> [!WARNING]
+> Gotify 3 isn't released yet.
+
+---
+
 - The `config.yml` file is no longer supported, convert it to the new env format
   with [`migrate-config`](#migrating-your-config).
 - If you set list or map environment variables, their syntax changed, see
@@ -33,7 +38,7 @@ $ gotify-server migrate-config config.yml > gotify-server.env
 With Docker:
 
 ```bash
-$ docker run --rm -v "$(pwd)/config.yml:/app/config.yml" gotify/server \
+$ docker run --rm -v "$(pwd)/config.yml:/app/config.yml" gotify/server:master \
     migrate-config config.yml > gotify-server.env
 ```