diff --git a/engine/default.go b/engine/default.go
index 41fe236e..188ae208 100644
--- a/engine/default.go
+++ b/engine/default.go
@@ -37,6 +37,10 @@ func RegisterMounter(runtime, name string, mounter runtime.Mounter) {
 	defaultEngine.RegisterMounter(runtime, name, mounter)
 }
 
+func RegisterRuntime(rt runtime.Runtime) {
+	defaultEngine.RegisterRuntime(rt)
+}
+
 func RegisterDatastoreProvider(name string, provider datastore.Provider) {
 	defaultEngine.RegisterDatastoreProvider(name, provider)
 }
diff --git a/engine/engine.go b/engine/engine.go
index 7cdf0443..93446022 100644
--- a/engine/engine.go
+++ b/engine/engine.go
@@ -50,6 +50,7 @@ type Engine struct {
 	broker      mq.Broker
 	ds          datastore.Datastore
 	mounters    map[string]*runtime.MultiMounter
+	runtime     runtime.Runtime
 	coordinator *coordinator.Coordinator
 	worker      *worker.Worker
 	dsProviders map[string]datastore.Provider
@@ -281,6 +282,16 @@ func (e *Engine) RegisterMounter(rt string, name string, mounter runtime.Mounter
 	mounters.RegisterMounter(name, mounter)
 }
 
+func (e *Engine) RegisterRuntime(rt runtime.Runtime) {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	e.mustState(StateIdle)
+	if e.runtime != nil {
+		panic("engine: RegisterRuntime called twice")
+	}
+	e.runtime = rt
+}
+
 func (e *Engine) RegisterDatastoreProvider(name string, provider datastore.Provider) {
 	e.mu.Lock()
 	defer e.mu.Unlock()
diff --git a/engine/engine_test.go b/engine/engine_test.go
index d8b053e9..fea9d0d5 100644
--- a/engine/engine_test.go
+++ b/engine/engine_test.go
@@ -10,6 +10,7 @@ import (
 	"github.com/runabol/tork"
 	"github.com/runabol/tork/datastore"
 	"github.com/runabol/tork/input"
+	"github.com/runabol/tork/runtime/shell"
 
 	"github.com/runabol/tork/mq"
 	"github.com/runabol/tork/runtime"
@@ -224,6 +225,20 @@ func TestRegisterMounter(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestRegisterRuntime(t *testing.T) {
+	eng := New(Config{Mode: ModeStandalone})
+	assert.Equal(t, StateIdle, eng.state)
+
+	eng.RegisterRuntime(shell.NewShellRuntime(shell.Config{}))
+
+	err := eng.Start()
+	assert.NoError(t, err)
+	assert.Equal(t, StateRunning, eng.state)
+
+	err = eng.Terminate()
+	assert.NoError(t, err)
+}
+
 func TestRegisterDatastoreProvider(t *testing.T) {
 	eng := New(Config{Mode: ModeStandalone})
 	assert.Equal(t, StateIdle, eng.state)
diff --git a/engine/worker.go b/engine/worker.go
index e0c2d916..9c061a0b 100644
--- a/engine/worker.go
+++ b/engine/worker.go
@@ -38,6 +38,9 @@ func (e *Engine) initWorker() error {
 }
 
 func (e *Engine) initRuntime() (runtime.Runtime, error) {
+	if e.runtime != nil {
+		return e.runtime, nil
+	}
 	runtimeType := conf.StringDefault("runtime.type", runtime.Docker)
 	switch runtimeType {
 	case runtime.Docker:
diff --git a/runtime/shell/shell.go b/runtime/shell/shell.go
index 92d15774..bd65512b 100644
--- a/runtime/shell/shell.go
+++ b/runtime/shell/shell.go
@@ -52,6 +52,12 @@ func NewShellRuntime(cfg Config) *ShellRuntime {
 	if cfg.Rexec == nil {
 		cfg.Rexec = reexec.Command
 	}
+	if cfg.UID == "" {
+		cfg.UID = DEFAULT_UID
+	}
+	if cfg.GID == "" {
+		cfg.GID = DEFAULT_GID
+	}
 	return &ShellRuntime{
 		cmds:   new(syncx.Map[string, *exec.Cmd]),
 		shell:  cfg.CMD,