docs: update tips for using custom SHELL in assemble, Fix #1633 (#1825)

Author: koldle

docs/usage/distrobox-assemble.md

@@ -207,3 +207,9 @@ each option corresponds to one of the `create` flags.
 	replace=true
 	start_now=true
 	pre_init_hooks=curl -fsSL https://bun.sh/install | bash;
+
+**Custom login shell example**
+
+	[ubuntu]
+	image=ubuntu:latest
+	pre_init_hooks="export SHELL=/bin/bash;"

docs/useful_tips.md

@@ -86,6 +86,13 @@ SHELL=/bin/zsh distrobox create -n test
 SHELL=/bin/zsh distrobox enter test
 ```
 
+If you want to declaratively set a custom shell for each container when using `distrobox assemble`,
+you can achieve this by using the `pre_init_hooks` option. For example:
+
+```ini
+pre_init_hooks="export SHELL=/bin/bash;"
+```
+
 ## Run the container with real root
 
 When using podman, distrobox will prefer to use rootless containers. In this mode the `root`