ci update (#108)

motivation: 5.6 is out

changes:
* use release version of 5.6
* add docker setup for 5.7 (using nightly for now)

docker/docker-compose.2004.56.yaml

@@ -6,7 +6,8 @@ services:
     image: swift-metrics:20.04-5.6
     build:
       args:
-        base_image: "swiftlang/swift:nightly-5.6-focal"
+        ubuntu_version: "focal"
+        swift_version: "5.6"
 
   test:
     image: swift-metrics:20.04-5.6

docker/docker-compose.2004.57.yaml

@@ -0,0 +1,17 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-metrics:20.04-5.7
+    build:
+      args:
+        base_image: "swiftlang/swift:nightly-main-focal"
+
+  test:
+    image: swift-metrics:20.04-5.7
+    environment: []
+      #- SANITIZER_ARG=--sanitize=thread
+
+  shell:
+    image: swift-metrics:20.04-5.7