add a config for Github codespaces

.devcontainer/devcontainer.json

@@ -0,0 +1,29 @@
+// For format details, see https://aka.ms/devcontainer.json. For config options, see the
+// README at: https://github.com/devcontainers/templates/tree/main/src/java
+{
+  "name": "Java",
+  // Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
+  "image": "mcr.microsoft.com/devcontainers/java:0-17",
+  "features": {
+    "ghcr.io/devcontainers/features/java:1": {
+      "version": "none",
+      "installMaven": "false",
+      "installGradle": "true"
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "vscjava.vscode-gradle"
+      ]
+    }
+  },
+  // Use 'forwardPorts' to make a list of ports inside the container available locally.
+  // "forwardPorts": [],
+  // Use 'postCreateCommand' to run commands after the container is created.
+  "postCreateCommand": "java -version"
+  // Configure tool-specific properties.
+  // "customizations": {},
+  // Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
+  // "remoteUser": "root"
+}