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

landing.fish

@@ -0,0 +1,33 @@
+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."
+end
+
+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