engine: resources: docker: Add a docker container resource

examples/lang/docker_container0.mcl

@@ -0,0 +1,6 @@
+docker:container "mgmt-nginx" {
+    state => "running",
+    image => "nginx",
+    cmd => ["nginx", "-g", "daemon off;",],
+    ports => {"tcp" => {80 => 8080,},},
+}