docker: use 5.4 release instead of nightly (#291)

Author: weissi

docker/docker-compose.2004.54.yaml

@@ -6,7 +6,7 @@ services:
     image: swift-nio-ssl:20.04-5.4
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.4-focal"
+        base_image: "swift:5.4-focal"
         ubuntu_version: "focal"
         swift_version: "5.4"