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# Docker backend

This is the original backend used with Woodpecker. The docker backend executes each step inside a separate container started on the agent.

## Docker credentials

Woodpecker supports [Docker credentials](https://github.com/docker/docker-credential-helpers) to securely store registry credentials. Install your corresponding credential helper and configure it in your Docker config file passed via [`WOODPECKER_DOCKER_CONFIG`](../10-server-config.md#woodpecker_docker_config).

To add your credential helper to the Woodpecker server container you could use the following code to build a custom image:

```dockerfile
FROM woodpeckerci/woodpecker-server:latest-alpine

RUN apk add -U --no-cache docker-credential-ecr-login
```

## Step specific configuration

### Run user

By default the docker backend starts the step container without the `--user` flag. This means the step container will use the default user of the container. To change this behavior you can set the `user` backend option to the preferred user/group:

```yaml
steps:
  - name: example
    image: alpine
    commands:
      - whoami
    backend_options:
      docker:
        user: 65534:65534
```

The syntax is the same as the [docker run](https://docs.docker.com/engine/reference/run/#user) `--user` flag.

## Image cleanup

The agent **will not** automatically remove images from the host. This task should be managed by the host system. For example, you can use a cron job to periodically do clean-up tasks for the CI runner.

:::danger
The following commands **are destructive** and **irreversible** it is highly recommended that you test these commands on your system before running them in production via a cron job or other automation.
:::

### Remove all unused images

<!-- cspell:ignore trunc -->

```bash
docker image rm $(docker images --filter "dangling=true" -q --no-trunc)
```

### Remove Woodpecker volumes

```bash
docker volume rm $(docker volume ls --filter name=^wp_* --filter dangling=true  -q)
```

## Tips and tricks

### Podman

There is no official support for Podman, but one can try to set the environment variable `DOCKER_HOST` to point to the Podman socket. It might work. See also the [Blog posts](https://woodpecker-ci.org/blog).

## Configuration

### `WOODPECKER_BACKEND_DOCKER_NETWORK`

> Default: empty

Set to the name of an existing network which will be attached to all your pipeline containers (steps). Please be careful as this allows the containers of different pipelines to access each other!

### `WOODPECKER_BACKEND_DOCKER_ENABLE_IPV6`

> Default: `false`

Enable IPv6 for the networks used by pipeline containers (steps). Make sure you configured your docker daemon to support IPv6.

### `WOODPECKER_BACKEND_DOCKER_VOLUMES`

> Default: empty

List of default volumes separated by comma to be mounted to all pipeline containers (steps). For example to use custom CA
certificates installed on host and host timezone use `/etc/ssl/certs:/etc/ssl/certs:ro,/etc/timezone:/etc/timezone`.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_MEM_SWAP`

> Default: `0`

The maximum amount of memory a single pipeline container is allowed to swap to disk, configured in bytes. There is no limit if `0`.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_MEM`

> Default: `0`

The maximum amount of memory a single pipeline container can use, configured in bytes. There is no limit if `0`.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_SHM_SIZE`

> Default: `0`

The maximum amount of memory of `/dev/shm` allowed in bytes. There is no limit if `0`.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_CPU_QUOTA`

> Default: `0`

The number of microseconds per CPU period that the container is limited to before throttled. There is no limit if `0`.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_CPU_SHARES`

> Default: `0`

The relative weight vs. other containers.

### `WOODPECKER_BACKEND_DOCKER_LIMIT_CPU_SET`

> Default: empty

Comma-separated list to limit the specific CPUs or cores a pipeline container can use.

Example: `WOODPECKER_BACKEND_DOCKER_LIMIT_CPU_SET=1,2`