swift 5.1 docker setup (#111)

motivation: support swift 5.1

changes:
* add docker compose file for swift 5.1, will add CI job once merged
* update swiftformat version to 0.40.12

Author: tomerd

Diff for: docker/Dockerfile

@@ -29,7 +29,7 @@ RUN chmod 755 $HOME/.tools/symbolicate-linux-fatal
 
 # swiftformat (until part of the toolchain)
 
-ARG swiftformat_version=0.40.11
+ARG swiftformat_version=0.40.12
 RUN git clone --branch $swiftformat_version --depth 1 https://github.com/nicklockwood/SwiftFormat $HOME/.tools/swift-format
 RUN cd $HOME/.tools/swift-format && swift build -c release
 RUN ln -s $HOME/.tools/swift-format/.build/release/swiftformat $HOME/.tools/swiftformat

Diff for: docker/docker-compose.1804.51.yaml

@@ -0,0 +1,13 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: async-http-client:18.04-5.1
+    build:
+      args:
+        ubuntu_version: "bionic"
+        swift_version: "5.1"
+
+  test:
+    image: async-http-client:18.04-5.1