switch to 5.3 nightly now that it is published (#164)

motivation: stable 5.3 ci

changes: pin 5.3 ci jobs to 5.3 nightlies instead of master

Author: tomerd

docker/docker-compose.al2.53.yaml

@@ -6,7 +6,7 @@ services:
     image: swift-aws-lambda:al2-5.3
     build:
       args:
-        base_image: "swiftlang/swift:nightly-amazonlinux2"
+        base_image: "swiftlang/swift:nightly-5.3-amazonlinux2"
 
   test:
     image: swift-aws-lambda:al2-5.3