set min engine version needed for v38 schema support

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

compose/const.py

@@ -43,6 +43,8 @@
 COMPOSEFILE_V3_7 = ComposeVersion('3.7')
 COMPOSEFILE_V3_8 = ComposeVersion('3.8')
 
+# minimum DOCKER ENGINE API version needed to support
+# features for each compose schema version
 API_VERSIONS = {
     COMPOSEFILE_V1: '1.21',
     COMPOSEFILE_V2_0: '1.22',
@@ -58,7 +60,7 @@
     COMPOSEFILE_V3_5: '1.30',
     COMPOSEFILE_V3_6: '1.36',
     COMPOSEFILE_V3_7: '1.38',
-    COMPOSEFILE_V3_8: '1.40',
+    COMPOSEFILE_V3_8: '1.38',
 }
 
 API_VERSION_TO_ENGINE_VERSION = {
@@ -76,5 +78,5 @@
     API_VERSIONS[COMPOSEFILE_V3_5]: '17.06.0',
     API_VERSIONS[COMPOSEFILE_V3_6]: '18.02.0',
     API_VERSIONS[COMPOSEFILE_V3_7]: '18.06.0',
-    API_VERSIONS[COMPOSEFILE_V3_8]: '19.03.0',
+    API_VERSIONS[COMPOSEFILE_V3_8]: '18.06.0',
 }