Escape dollar sign in serialized config output

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

compose/config/serialize.py

@@ -21,8 +21,11 @@ def serialize_dict_type(dumper, data):
 
 
 def serialize_string(dumper, data):
-    """ Ensure boolean-like strings are quoted in the output """
+    """ Ensure boolean-like strings are quoted in the output and escape $ characters """
     representer = dumper.represent_str if six.PY3 else dumper.represent_unicode
+
+    data = data.replace('$', '$$')
+
     if data.lower() in ('y', 'n', 'yes', 'no', 'on', 'off', 'true', 'false'):
         # Empirically only y/n appears to be an issue, but this might change
         # depending on which PyYaml version is being used. Err on safe side.

tests/unit/config/config_test.py

@@ -4185,3 +4185,25 @@ class SerializeTest(unittest.TestCase):
         assert 'command: "true"\n' in serialized_config
         assert 'FOO: "Y"\n' in serialized_config
         assert 'BAR: "on"\n' in serialized_config
+
+    def test_serialize_escape_dollar_sign(self):
+        cfg = {
+            'version': '2.2',
+            'services': {
+                'web': {
+                    'image': 'busybox',
+                    'command': 'echo $$FOO',
+                    'environment': {
+                        'CURRENCY': '$$'
+                    },
+                    'entrypoint': ['$$SHELL', '-c'],
+                }
+            }
+        }
+        config_dict = config.load(build_config_details(cfg))
+
+        serialized_config = yaml.load(serialize_config(config_dict))
+        serialized_service = serialized_config['services']['web']
+        assert serialized_service['environment']['CURRENCY'] == '$$'
+        assert serialized_service['command'] == 'echo $$FOO'
+        assert serialized_service['entrypoint'][0] == '$$SHELL'