Update Swift 5.7 docker image (#37)

Author: yim-lee

docker/docker-compose.2004.57.yaml

@@ -6,7 +6,7 @@ services:
     image: swift-statsd-client:20.04-5.7
     build:
       args:
-        base_image: "swiftlang/swift:nightly-main-focal"
+        base_image: "swiftlang/swift:nightly-5.7-focal"
 
   test:
     image: swift-statsd-client:20.04-5.7