# Customize Ubuntu 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 application's requirements. This is an optional phase in the [build and deploy pipeline](/features/pipeline#runtime-prepare-phase-optional).

Ubuntu is a versatile base for running anything not explicitly offered as a dedicated Zerops runtime. You can install any packages and tools you need, treating it as a clean OS to customize however you want.

It is also a great option when you need a specific version of a technology (like Go, Node.js, or PHP) that Zerops doesn't support by default—whether it's an older version for legacy projects or a newer release not yet available.

## Configuration

### Default Ubuntu Runtime Environment

The default Ubuntu runtime environment contains:

- <span>Ubuntu 24.04</span>
- [zCLI](/references/cli)
- Git

### When You Need a Custom Runtime Image

Since Ubuntu serves as a general-purpose base, you'll likely want to customize it for your specific use case. Common scenarios include:

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

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

### Basic Ubuntu Setup

### Using Build Files in Runtime Preparation

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](/ubuntu/how-to/logs#prepare-runtime-log) for specific error messages.