Add user as docker backend_option (#4526)

pipeline/backend/docker/backend_options.go

@@ -0,0 +1,21 @@
+package docker
+
+import (
+	"github.com/mitchellh/mapstructure"
+
+	backend "go.woodpecker-ci.org/woodpecker/v2/pipeline/backend/types"
+)
+
+// BackendOptions defines all the advanced options for the docker backend.
+type BackendOptions struct {
+	User string `mapstructure:"user"`
+}
+
+func parseBackendOptions(step *backend.Step) (BackendOptions, error) {
+	var result BackendOptions
+	if step == nil || step.BackendOptions == nil {
+		return result, nil
+	}
+	err := mapstructure.Decode(step.BackendOptions[EngineName], &result)
+	return result, err
+}