Don't break when interpolating environment with unicode characters

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/acceptance/cli_test.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 from __future__ import absolute_import
 from __future__ import unicode_literals
 
@@ -12,6 +13,7 @@ from collections import namedtuple
 from operator import attrgetter
 
 import py
+import six
 import yaml
 from docker import errors
 
@@ -1286,6 +1288,23 @@ class CLITestCase(DockerClientTestCase):
             'simplecomposefile_simple_run_1',
             'exited'))
 
+    @mock.patch.dict(os.environ)
+    def test_run_unicode_env_values_from_system(self):
+        value = 'ą, ć, ę, ł, ń, ó, ś, ź, ż'
+        if six.PY2:  # os.environ doesn't support unicode values in Py2
+            os.environ['BAR'] = value.encode('utf-8')
+        else:  # ... and doesn't support byte values in Py3
+            os.environ['BAR'] = value
+        self.base_dir = 'tests/fixtures/unicode-environment'
+        result = self.dispatch(['run', 'simple'])
+
+        if six.PY2:  # Can't retrieve output on Py3. See issue #3670
+            assert value == result.stdout.strip()
+
+        container = self.project.containers(one_off=OneOffFilter.only, stopped=True)[0]
+        environment = container.get('Config.Env')
+        assert 'FOO={}'.format(value) in environment
+
     @mock.patch.dict(os.environ)
     def test_run_env_values_from_system(self):
         os.environ['FOO'] = 'bar'