--bind, --chdir and relative destination paths

Hello, I was a bit suprised by the following behaviour and I wondered if this was a WONTFIX or if my surprise was legitimate and improvable.
If I use `--dir "$PWD" --chdir "$PWD" --ro-bind aaa aaa`, I expect `aaa` to be bound in `"$PWD"`, but it's bound in `/`. I could understand not wanting to complicate things by mixing the CWD and paths, but who knows.

Test case:
```
$ cd
$ touch aaa
$ bwrap --ro-bind /bin /bin --ro-bind /usr/bin /usr/bin --ro-bind /usr/lib /usr/lib --ro-bind /lib64 /lib64 --dir "$PWD" --chdir "$PWD" --ro-bind aaa aaa sh -c 'ls -l; ls -l /'
total 0
total 16
-rw-r--r-- 1 1000 1000 0 Apr 10 11:15 aaa
drwxr-xr-x 2 0 0 4096 Apr 8 20:56 bin
drwxr-xr-x 3 1000 1000 60 Apr 10 11:16 home
drwxr-xr-x 6 0 0 8192 Apr 8 20:56 lib64
drwx------ 4 1000 1000 80 Apr 10 11:16 usr
```