docker-cli: add docker contexts support

Adds docker-cli.contexts support. This allows declarative configuration
of [docker
contexts](https://docs.docker.com/engine/manage-resources/contexts/).

tests/modules/programs/docker-cli/example-contexts.json

@@ -0,0 +1,11 @@
+{
+  "Endpoints": {
+    "docker": {
+      "Host": "unix://example2"
+    }
+  },
+  "Metadata": {
+    "Description": "example1"
+  },
+  "Name": "example"
+}