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.

dbschemas/02-posts-add-updated.sql

@@ -0,0 +1,3 @@
+-- This SQL script adds a new column 'updated_at' to the 'posts' table.
+ALTER TABLE posts
+ADD COLUMN updated_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP;