run docker step in host mode (#857)

## Problem
In host executor mode, uses: docker://<image> step actions fail because
act/runner/step_docker.go always attaches the step container to the job
container's network namespace, which doesn't exist in host mode.

### Example
Run following job in host runner

```yaml
jobs:
  test:
    runs-on: ubuntu-latest-host
    steps:
      - uses: docker://alpine:3.20
        with:
          args: echo hello
```
```
Error:
  failed to start container: Error response from daemon:
    joining network namespace of container:
    No such container: xxxxxx
```

This pr allows the docker step in the host mode

## Testing
I tested following steps on host runner and it worked

```yaml

 - name: Test azure cli action in host mode
   uses: azure/cli@v2
   env:
     RUNNER_OS: Linux
   with:
     inlineScript: echo "hello from azure cli"
 
 - uses: docker://alpine:3.20
   with:
     args: echo hello

```

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/857
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: Mirko Sekulic <misha.sekulic@gmail.com>
Co-committed-by: Mirko Sekulic <misha.sekulic@gmail.com>

act/runner/step_docker.go

@@ -116,6 +116,10 @@ func (sd *stepDocker) newStepContainer(ctx context.Context, image string, cmd, e
 	envList = append(envList, fmt.Sprintf("%s=%s", "RUNNER_TEMP", "/tmp"))
 
 	binds, mounts := rc.GetBindsAndMounts()
+	networkMode := "container:" + rc.jobContainerName()
+	if rc.IsHostEnv(ctx) {
+		networkMode = "default"
+	}
 	stepContainer := ContainerNewContainer(&container.NewContainerInput{
 		Cmd:          cmd,
 		Entrypoint:   entrypoint,
@@ -126,7 +130,7 @@ func (sd *stepDocker) newStepContainer(ctx context.Context, image string, cmd, e
 		Name:         createSimpleContainerName(rc.jobContainerName(), "STEP-"+step.ID),
 		Env:          envList,
 		Mounts:       mounts,
-		NetworkMode:  "container:" + rc.jobContainerName(),
+		NetworkMode:  networkMode,
 		Binds:        binds,
 		Stdout:       logWriter,
 		Stderr:       logWriter,

act/runner/step_docker_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"context"
 	"io"
+	"strings"
 	"testing"
 
 	"gitea.com/gitea/act_runner/act/container"
@@ -118,3 +119,81 @@ func TestStepDockerPrePost(t *testing.T) {
 	err = sd.post()(ctx)
 	assert.Nil(t, err) //nolint:testifylint // pre-existing issue from nektos/act
 }
+
+func TestStepDockerNewStepContainerNetworkMode(t *testing.T) {
+	cases := []struct {
+		name          string
+		platform      string
+		expectDefault bool
+	}{
+		{
+			name:          "docker mode attaches to job container network",
+			platform:      "node:14",
+			expectDefault: false,
+		},
+		{
+			name:          "host mode uses default network",
+			platform:      "-self-hosted",
+			expectDefault: true,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			cm := &containerMock{}
+
+			var captured *container.NewContainerInput
+			origContainerNewContainer := ContainerNewContainer
+			ContainerNewContainer = func(input *container.NewContainerInput) container.ExecutionsEnvironment {
+				captured = input
+				return cm
+			}
+			defer func() {
+				ContainerNewContainer = origContainerNewContainer
+			}()
+
+			ctx := context.Background()
+
+			platform := tc.platform
+			sd := &stepDocker{
+				RunContext: &RunContext{
+					StepResults: map[string]*model.StepResult{},
+					Config: &Config{
+						PlatformPicker: func(_ []string) string {
+							return platform
+						},
+					},
+					Run: &model.Run{
+						JobID: "1",
+						Workflow: &model.Workflow{
+							Jobs: map[string]*model.Job{
+								"1": {},
+							},
+						},
+					},
+					JobContainer: cm,
+				},
+				Step: &model.Step{
+					ID:   "1",
+					Uses: "docker://alpine:3.20",
+				},
+			}
+			sd.RunContext.ExprEval = sd.RunContext.NewExpressionEvaluator(ctx)
+
+			assert.Equal(t, tc.expectDefault, sd.RunContext.IsHostEnv(ctx),
+				"IsHostEnv mismatch for platform %q", tc.platform)
+
+			_ = sd.newStepContainer(ctx, "alpine:3.20", []string{"echo", "hello"}, nil)
+
+			if tc.expectDefault {
+				assert.Equal(t, "default", captured.NetworkMode,
+					"host-mode step container must use 'default' network, got %q",
+					captured.NetworkMode)
+			} else {
+				assert.True(t, strings.HasPrefix(captured.NetworkMode, "container:"),
+					"docker-mode step container must attach to job container network, got %q",
+					captured.NetworkMode)
+			}
+		})
+	}
+}