Add devcontainer configuration for GitHub Codespaces

.devcontainer/Dockerfile

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+# Use latest Ubuntu as base
+FROM ubuntu:latest
+
+# Avoid warnings by switching to noninteractive
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Configure apt and install packages
+RUN apt-get update \
+    && apt-get -y install --no-install-recommends \
+    # Build essentials and tools
+    build-essential \
+    cmake \
+    ninja-build \
+    pkg-config \
+    git \
+    # Required dependencies from GitHub Actions workflow
+    libssl-dev \
+    sqlite3 \
+    libsqlite3-dev \
+    libcurl4 \
+    libcurl4-openssl-dev \
+    uuid-dev \
+    libgtest-dev \
+    # C++ development and debugging tools
+    gdb \
+    gdbserver \
+    valgrind \
+    clang \
+    clang-format \
+    clang-tidy \
+    lldb \
+    # Additional utilities
+    curl \
+    wget \
+    vim \
+    nano \
+    htop \
+    tree \
+    zip \
+    unzip \
+    ca-certificates \
+    sudo \
+    # Clean up
+    && apt-get autoremove -y \
+    && apt-get clean -y \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create a non-root user to use
+ARG USERNAME=vscode
+ARG USER_UID=1000
+ARG USER_GID=$USER_UID
+
+RUN groupadd --gid $USER_GID $USERNAME \
+    && useradd --uid $USER_UID --gid $USER_GID -m $USERNAME \
+    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME \
+    && chmod 0440 /etc/sudoers.d/$USERNAME
+
+# Switch back to dialog for any ad-hoc use of apt-get
+ENV DEBIAN_FRONTEND=dialog
+
+# Set the default user
+USER $USERNAME

.devcontainer/README.md

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+# SciTokens C++ Development Container
+
+This devcontainer configuration enables GitHub Codespaces and local development container support for the scitokens-cpp project.
+
+## Features
+
+### Base Environment
+- **OS**: Latest Ubuntu
+- **User**: Non-root `vscode` user with sudo access
+
+### Build Dependencies
+All dependencies from the GitHub Actions workflow are included:
+- `libssl-dev` - OpenSSL development files
+- `sqlite3` and `libsqlite3-dev` - SQLite database
+- `cmake` - Build system
+- `libcurl4` and `libcurl4-openssl-dev` - HTTP client library
+- `uuid-dev` - UUID generation library
+- `libgtest-dev` - Google Test framework
+
+### Development Tools
+- **Build Tools**: build-essential, cmake, ninja-build, pkg-config
+- **Debuggers**: gdb, gdbserver, lldb
+- **Analysis Tools**: valgrind, clang-tidy
+- **Compilers**: gcc/g++ (via build-essential), clang
+- **Formatters**: clang-format
+- **Utilities**: git, curl, wget, vim, nano, htop, tree
+
+### VSCode Extensions
+- C/C++ Extension Pack
+- CMake Tools
+- CMake Language Support
+- GitHub Copilot (if available)
+
+## Usage
+
+### GitHub Codespaces
+1. Navigate to the repository on GitHub
+2. Click the "Code" button
+3. Select "Codespaces" tab
+4. Click "Create codespace on [branch]"
+
+### Local Development with VSCode
+1. Install [Docker](https://www.docker.com/products/docker-desktop)
+2. Install [VSCode](https://code.visualstudio.com/)
+3. Install the [Dev Containers extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers)
+4. Open the repository in VSCode
+5. Click "Reopen in Container" when prompted (or use Command Palette: "Dev Containers: Reopen in Container")
+
+## Building the Project
+
+After the container is created, submodules are automatically initialized. To build:
+
+```bash
+# Create build directory
+mkdir -p build
+cd build
+
+# Configure with CMake
+cmake .. -DCMAKE_BUILD_TYPE=Release -DSCITOKENS_BUILD_UNITTESTS=ON
+
+# Build
+cmake --build .
+
+# Run tests
+ctest --verbose
+```
+
+## Debugging
+
+The container includes both GDB and LLDB debuggers. You can:
+- Use VSCode's integrated debugging features
+- Run debuggers from the terminal
+- Use valgrind for memory analysis
+
+Example using GDB:
+```bash
+gdb ./scitokens-test
+```

.devcontainer/devcontainer.json

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+{
+	"name": "SciTokens C++ Development",
+	"build": {
+		"dockerfile": "Dockerfile"
+	},
+
+	// Configure tool-specific properties.
+	"customizations": {
+		"vscode": {
+			"settings": {
+				"terminal.integrated.defaultProfile.linux": "bash",
+				"C_Cpp.default.configurationProvider": "ms-vscode.cmake-tools"
+			},
+			"extensions": [
+				"ms-vscode.cpptools",
+				"ms-vscode.cpptools-extension-pack",
+				"ms-vscode.cmake-tools",
+				"twxs.cmake",
+				"github.copilot",
+				"github.copilot-chat"
+			]
+		}
+	},
+
+	// 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": "git submodule update --init --recursive",
+
+	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
+	"remoteUser": "vscode",
+
+	"features": {
+		"ghcr.io/devcontainers/features/git:1": {}
+	}
+}