Add devcontainer configuration to reduce Copilot agent build time from 5+ minutes to under 1 minute

.devcontainer/devcontainer.json

@@ -0,0 +1,26 @@
+{
+  "name": "Application Insights Java Development Environment",
+  "image": "mcr.microsoft.com/vscode/devcontainers/base:bullseye",
+  "features": {
+    "ghcr.io/devcontainers/features/java:1": {
+      "version": "17"
+    },
+    "ghcr.io/devcontainers/features/docker-in-docker:2": {
+      "version": "latest"
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "vscjava.vscode-java-pack",
+        "vscjava.vscode-gradle",
+        "ms-vscode.vscode-json"
+      ]
+    }
+  },
+  "mounts": [
+    "source=gradle-cache,target=/home/vscode/.gradle,type=volume"
+  ],
+  "postCreateCommand": ".devcontainer/setup-devcontainer.sh",
+  "remoteUser": "vscode"
+}

.devcontainer/setup-devcontainer.sh

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+#!/bin/bash
+set -e
+
+echo "Setting up ApplicationInsights-Java development environment..."
+
+# GRADLE_OPTS is already set via containerEnv in devcontainer.json
+
+# Pre-download Gradle wrapper and dependencies
+echo "Pre-downloading Gradle wrapper and dependencies..."
+./gradlew --version
+
+# Build project to download dependencies and populate build cache
+echo "Building project to download dependencies and populate build cache..."
+./gradlew build --quiet || true
+
+echo "Development environment setup complete!"