Update to Swift 5.5 release in CI (#14)

Author: Lukasa

docker/docker-compose.2004.55.yaml

@@ -6,7 +6,7 @@ services:
     image: swift-http-structured-headers:20.04-5.5
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.5-focal"
+        swift_version: "5.5"
         ubuntu_version: "focal"
 
   unit-tests: