# Customise Deno runtime environment


## Build Custom Runtime Images

Zerops allows you to build custom runtime images (CRI) when the default base runtime images don't meet your Deno application's requirements. This is an optional phase in the [build and deploy pipeline](/features/pipeline#runtime-prepare-phase-optional).

:::important
You should not include your application code in the custom runtime image, as your built/packaged code is deployed automatically into fresh containers.
:::

## Configuration

### Default Deno Runtime Environment

The default Deno runtime environment contains:

- <span>Ubuntu 24.04</span>
- Selected version of Deno when the runtime service was created
- [zCLI](/references/cli)
- Deno and Git

### When You Need a Custom Runtime Image

If your Deno application needs more than what's included in the default environment, you'll need to build a custom runtime image. Common scenarios include:

- **System packages for processing**: When your app processes images, videos, or files (requiring packages like `sudo apt-get install -y imagemagick`)
- **Global Deno tools**: When you need CLI tools or utilities available system-wide
- **Native dependencies**: When your Deno modules require system libraries that aren't in the default environment

Here are Deno-specific examples of configuring custom runtime images in your `zerops.yml`:

### Basic Deno Setup

### Using Build Files in Runtime Preparation

```yaml
build:
  addToRunPrepare:
    - deno.json
    - import_map.json
run:
  prepareCommands:
    - sudo apt-get update
    - sudo apt-get install -y imagemagick
    - deno cache deps.ts
```

For complete configuration details, see the [runtime prepare phase configuration guide](/features/pipeline#configuration).

## Process and Caching

### How Runtime Prepare Works
The runtime prepare process follows the same steps for all runtimes. See [how runtime prepare works](/features/pipeline#how-it-works) for the complete process details.

### Caching Behavior
Zerops caches custom runtime images to optimize deployment times. Learn about [custom runtime image caching](/features/pipeline#custom-runtime-image-caching) including when images are cached and reused.

### Build Management
For information about managing builds and deployments, see [managing builds and deployments](/features/pipeline#manage-builds-and-deployments).

:::warning
Shared storage mounts are not available during the runtime prepare phase.
:::

## Troubleshooting

If your `prepareCommands` fail, check the [prepare runtime log](/deno/how-to/logs#prepare-runtime-log) for specific error messages.