Add option to set default volumes for docker backend (#1203)

docs/docs/30-administration/22-backends/10-docker.md

@@ -14,6 +14,12 @@ Set to the name of an existing network which will be attached to all your pipeli
 
 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`.
+
 ## 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).
@@ -28,4 +34,4 @@ RUN apk add -U --no-cache docker-credential-ecr-login
 
 ## Podman support
 
-While the agent was developped with Docker/Moby, Podman can also be used by setting the environment variable `DOCKER_SOCK` to point to the podman socket. In order to work without workarounds, Podman 4.0 (or above) is required.
+While the agent was developed with Docker/Moby, Podman can also be used by setting the environment variable `DOCKER_SOCK` to point to the podman socket. In order to work without workarounds, Podman 4.0 (or above) is required.

pipeline/backend/docker/docker.go

@@ -23,6 +23,7 @@ type docker struct {
 	client     client.APIClient
 	enableIPv6 bool
 	network    string
+	volumes    []string
 }
 
 // make sure docker implements Engine
@@ -59,6 +60,18 @@ func (e *docker) Load() error {
 
 	e.network = os.Getenv("WOODPECKER_BACKEND_DOCKER_NETWORK")
 
+	volumes := strings.Split(os.Getenv("WOODPECKER_BACKEND_DOCKER_VOLUMES"), ",")
+	e.volumes = make([]string, 0, len(volumes))
+	// Validate provided volume definitions
+	for _, v := range volumes {
+		parts, err := splitVolumeParts(v)
+		if err != nil {
+			log.Error().Err(err).Msgf("invalid volume '%s' provided in WOODPECKER_BACKEND_DOCKER_VOLUMES", v)
+			continue
+		}
+		e.volumes = append(e.volumes, strings.Join(parts, ":"))
+	}
+
 	return nil
 }
 
@@ -117,6 +130,9 @@ func (e *docker) Exec(ctx context.Context, proc *backend.Step) error {
 		}
 	}
 
+	// add default volumes to the host configuration
+	hostConfig.Binds = append(hostConfig.Binds, e.volumes...)
+
 	_, err := e.client.ContainerCreate(ctx, config, hostConfig, nil, nil, proc.Name)
 	if client.IsErrNotFound(err) {
 		// automatically pull and try to re-create the image if the