DevContainer support (#340)

Add `.devcontainer` files to enable using [VS Code Dev
Containers](https://code.visualstudio.com/docs/devcontainers/containers)
and/or [GitHub Codespaces](https://github.com/features/codespaces).

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: devhawk

.devcontainer/README.md

@@ -0,0 +1,29 @@
+# DBOS Transact Java - Development Container
+
+Simple devcontainer configuration for GitHub Codespaces and VS Code Dev Containers.
+
+## What's Included
+
+- **Java 21**: Pre-installed and ready to use
+- **Docker-in-Docker**: For running Testcontainers in tests
+- **VS Code Extensions**: Java Extension Pack
+
+## Quick Start
+
+### GitHub Codespaces
+1. Click "Code" → "Codespaces" → "Create codespace"
+2. Run `./gradlew build` to build the project
+3. Run `./gradlew test` to run tests
+
+### VS Code Dev Containers (Local)
+1. Open project in VS Code with "Dev Containers" extension installed
+2. Select "Reopen in Container" when prompted
+3. Run `./gradlew build` to build the project
+
+## Testing
+
+Tests use **Testcontainers** to automatically spin up PostgreSQL instances - no manual setup needed!
+
+```bash
+./gradlew test
+```

.devcontainer/devcontainer.json

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+{
+  "name": "Java 21 Bare Minimum",
+  "image": "mcr.microsoft.com/devcontainers/java:21",
+  "features": {
+    "ghcr.io/devcontainers/features/docker-in-docker:2": {
+      "moby": false
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": ["vscjava.vscode-java-pack"]
+    }
+  },
+  "remoteUser": "vscode"
+}