Initial exercise setup.

Additions

* Instructions for manually starting the database.
* Each phase now has a success criteria with how to verify.
* Troubleshooting section for working with the Docker DB container.
* All phases of the exercise are described in README.md (with as little advanced hints as possible).
* DFD to illustrate phase 1
* Debugging starts the database container and applies schema migrations (no setup required).
* Launch settings for VS Code, VS 2022 and Rider (IntelliJ IDEA).
* Swagger UI is available at <http://localhost:8080/swagger/index.html> for testing.

service/PostModel.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace service;
+
+[Serializable]
+public class PostModel
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public string Title { get; set; }
+    public string Body { get; set; }
+}