feat: overhaul container initialization and entrypoint

- Replace apt-get with apt in Dockerfile
- Add tini as init system and set ENTRYPOINT/CMD
- Simplify init.fish by removing first-run logic and tmux session management
- Add landing.fish and SIGTERM signal handling
- Improve container lifecycle management

init.fish

@@ -1,12 +1,4 @@
 function init
-	if test -f /tmp/bubble_container_started
-		return
-	end
-	echo "First login detected, initializing container environment..."
-	if ! touch /tmp/bubble_container_started
-		echo "FAILED to mark status. The initialization was cancelled."
-		echo "Certain services (like Tailscale) won't work."
-	end
 	if test -f /usr/sbin/tailscaled
 		echo "Starting tailscaled"
 		chmod +x /usr/sbin/tailscaled
@@ -24,37 +16,10 @@ function init
 end
 
 init
-
-if ! cat /etc/passwd | grep -E "^user"
-    echo "User was not created correctly. Creating user..."
-    useradd -m -s /usr/bin/fish user
-    if test ! -e /etc/sudoers.d/user.conf
-        echo "Setting sudo permissions"
-        mkdir -p /etc/sudoers.d/
-        echo "user ALL=(ALL) NOPASSWD: ALL" >/etc/sudoers.d/user.conf
-    end
-    echo "Due to abnormal user creation, auto configuration guide is not available in this setup."
-    echo "Try 'bubble destroy' for a new container if needed."
+function on_term --on-signal SIGTERM
+    echo "SIGTERM received, exiting"
+    exit
 end
+fish
 
-function resume
-    if test (sudo -u user tmux list-windows | wc -l) -eq 0
-		cd /home/user
-        sudo -u user tmux
-        return
-    end
-    if sudo -u user tmux has-session -t 0
-		#        if sudo -u user tmux ls | grep -E "^0.*attached)"
-		#            echo "The default tmux session has been attached, dropping you to a normal shell."
-		#            cd /home/user && sudo -u user /usr/bin/fish
-		#        else
-		#            sudo -u user tmux attach -t 0
-		#        end
-		sudo -u user tmux attach -t 0
-    else
-		cd /home/user
-        sudo -u user tmux new-session -c /usr/bin/fish
-    end
-end
 
-resume