Fixes #85 - Add devcontainer.json

Author: Automated workflow

.devcontainer/devcontainer.json

@@ -0,0 +1,11 @@
+{
+    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+    "features": {
+        "ghcr.io/devcontainers/features/docker-in-docker:2": {},
+        "ghcr.io/devcontainers/features/git:1": {},
+        "ghcr.io/devcontainers/features/java:1": {
+            "version": "21",
+            "installMaven": "true"
+        }
+    }
+}