improve 5.3 docker setup (#214)
improve 5.3 docker setup (#214)

motivation: pin to 5.3 nighties 

changes: use swiftlang/swift:nightly-5.3-master-bionic instead of swiftlang/swift:nightly-master-bionic
apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl