Write a Python function `join_integers` that takes a list of integers as input and returns a single integer formed by concatenating the digits of the integers in the list.
#### Requirements:
- The function should handle both positive and negative integers.
- The resulting integer should preserve the order of the input list.
#### Example Usage:
```python [main.nopy]
join_integers([11, 33, 50]) # Output: 113350
join_integers([-1, 2, 3, 4, 5, 6]) # Output: -123456
join_integers([10, 15, 20, 25]) # Output: 10152025
```