Use a local SQLite3 backend instead of PostgreSQL Docker container.

* The provided SQLite3 database contains the required schemas, but no data and can be reset to if required.
* Remove all references to PostgreSQL in documentation and configuration.
* Replace native sqlite3 command with a console app to remove dependency on SQLite3 installation.

show-posts/PostsDbContext.cs

@@ -0,0 +1,20 @@
+using Microsoft.EntityFrameworkCore;
+
+public class PostsDbContext : DbContext
+{
+    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+    {
+        var currentDir = System.IO.Directory.GetCurrentDirectory();
+        int showPostsIndex = currentDir.IndexOf("show-posts");
+        if (showPostsIndex > -1)
+        {
+            // Running from the bin directory, get the parent of "show-posts".
+            currentDir = currentDir.Substring(0, showPostsIndex - 1);
+        }
+        string repoRoot = currentDir;
+
+        optionsBuilder.UseSqlite($"Data Source={repoRoot}/service/service.db;Mode=ReadOnly;");
+    }
+
+    public DbSet<Post> Posts { get; set; }
+}