Add dev container

This is what VS Code suggests for Java development. Using `bookworm` (i.e. Debian 12) doesn't work for some reason, so we're sticking with `bullseye` (i.e. Debian 11). We don't need Gradle installed in the container, as we're using the Grade wrapper.

.devcontainer/devcontainer.json

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+// 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:1-8-bullseye",
+
+	"features": {
+		"ghcr.io/devcontainers/features/java:1": {
+			"version": "none",
+			"installMaven": "false",
+			"installGradle": "false"
+		}
+	}
+
+	// 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"
+}