Add Docker support and update Java version to 21

src/main/java/_11/asktpk/artisanconnectbackend/ArtisanConnectBackendApplication.java

@@ -14,7 +14,7 @@ public class ArtisanConnectBackendApplication {
         this.environment = environment;
     }
 
-    static void main(String[] args) {
+    public static void main(String[] args) {
         SpringApplication.run(ArtisanConnectBackendApplication.class, args);
     }