add devcontainer

this adds proper java support in github codespaces

.devcontainer/devcontainer.json

@@ -0,0 +1,24 @@
+// 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:1-21-bullseye",
+
+	"features": {
+		"ghcr.io/devcontainers/features/java:1": {},
+		"ghcr.io/devcontainers-contrib/features/maven-sdkman:2": {}
+	}
+
+	// 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"
+}