[UX] Support `~` for `path` in instance volumes

Currently, I have to type:

```
type: task

volumes:
  - /dstack-cache/pip:/root/.cache/pip
 ```

When we don't know the username, specifying home directory paths is very difficult.

Ideal would be:

```
type: task

volumes:
  - /dstack-cache/pip:~/.cache/pip
 ```