Add README documentation for devcontainer image

devcontainer/README.md

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+# Toolbox Devcontainer
+
+A development container image with git, SSH, GPG, and locale support. Built on the [toolbox base image](../base), which uses Debian 13 (slim) as its foundation.
+
+## Image Details
+
+- **Registry**: `ghcr.io/jhulten/toolbox/devcontainer`
+- **Base Image**: `ghcr.io/jhulten/toolbox/base`
+- **User**: `toolsmith`
+- **Working Directory**: `/home/toolsmith`
+
+## Included Tools
+
+- **git** - Version control
+- **openssh-client** - SSH client for remote connections
+- **gnupg** - GPG for signing commits and encryption
+- **mise** - Polyglot tool version manager (inherited from base image)
+- **Locale Support** - UTF-8 locale configured (`en_US.UTF-8`)
+
+## Usage
+
+### VS Code Dev Containers
+
+1. Install the [Dev Containers extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers)
+2. Copy the `devcontainer.json` file to your project's `.devcontainer/` directory:
+
+   ```bash
+   mkdir -p .devcontainer
+   cp devcontainer.json .devcontainer/
+   ```
+
+3. Modify the `devcontainer.json` to reference the pre-built image instead of building locally (replace the `build` section with `image`):
+
+   ```json
+   {
+     "name": "Toolbox Development Container",
+     "image": "ghcr.io/jhulten/toolbox/devcontainer:latest",
+     "remoteUser": "toolsmith",
+     "containerUser": "toolsmith",
+     "updateRemoteUserUID": true,
+     "customizations": {
+       "vscode": {
+         "settings": {
+           "terminal.integrated.defaultProfile.linux": "bash"
+         },
+         "extensions": []
+       }
+     }
+   }
+   ```
+
+   Alternatively, keep the original `devcontainer.json` to build the image locally from the Dockerfile.
+
+4. Open the Command Palette (`Ctrl+Shift+P` / `Cmd+Shift+P`) and select **Dev Containers: Reopen in Container**
+
+### GitHub Codespaces
+
+Create a `.devcontainer/devcontainer.json` file in your repository:
+
+```json
+{
+  "name": "Toolbox Development Container",
+  "image": "ghcr.io/jhulten/toolbox/devcontainer:latest",
+  "remoteUser": "toolsmith",
+  "containerUser": "toolsmith",
+  "updateRemoteUserUID": true
+}
+```
+
+Then open your repository in GitHub Codespaces.
+
+### Docker CLI
+
+Pull and run the image directly:
+
+```bash
+# Pull the latest image
+docker pull ghcr.io/jhulten/toolbox/devcontainer:latest
+
+# Run interactively
+docker run -it --rm ghcr.io/jhulten/toolbox/devcontainer:latest bash
+
+# Mount a local directory
+docker run -it --rm -v "$(pwd):/workspace" -w /workspace ghcr.io/jhulten/toolbox/devcontainer:latest bash
+```
+
+## Image Tags
+
+- `latest` - Built from the main branch
+- `sha-<commit>` - Built from a specific commit
+
+## Customization
+
+### Adding VS Code Extensions
+
+Add extensions to the `customizations.vscode.extensions` array in your `devcontainer.json`:
+
+```json
+{
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "GitHub.copilot",
+        "eamodio.gitlens"
+      ]
+    }
+  }
+}
+```
+
+### Installing Additional Tools with mise
+
+The base image includes [mise](https://mise.jdx.dev/) for managing tool versions. Add a `.mise.toml` file to your project:
+
+```toml
+[tools]
+node = "22"
+python = "3.12"
+go = "1.23"
+```
+
+Then run `mise install` in your dev container to install the specified tools.
+
+## License
+
+MIT - See [LICENSE](../LICENSE) for details.