Open a docker instance (e.g., `docker run -ti ubuntu:16.04`), create the Python files `a.py`

```
# -*- coding: utf-8 -*-
a = 'ö'

```

and `r.py`

```
with open('a.py') as f: exec(f.read())

```

When executing `python r.py`, all is file. When using `python3`, however, one gets the dreaded

```
Traceback (most recent call last):
  File "r.py", line 2, in <module>
    exec(f.read())
  File "/usr/lib/python3.5/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 31: ordinal not in range(128)

```

Curiously, I can reproduce this *only on docker*.

Any hints on what may be going wrong, and how to fix it? (Note that I can only modify `r.py`.)