Pass 'sleep' as container command rather than entrypoint (#86)

The current code overrides the container's entrypoint with `sleep`.  Unfortunately, that prevents initialization scripts, such as to initialize Docker-in-Docker, from running.

The change simply moves the `sleep` to the command, rather than entrypoint, directive.

For most containers of this sort, the entrypoint script performs initialization, and then ends with `$@` to execute whatever command is passed.

If the container has no entrypoint, the command is executed directly.  As a result, this should be a transparent change for most use cases, while allowing the container's entrypoint to be used when present.

Reviewed-on: https://gitea.com/gitea/act/pulls/86
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: Thomas E Lackey <telackey@bozemanpass.com>
Co-committed-by: Thomas E Lackey <telackey@bozemanpass.com>

pkg/runner/run_context.go

@@ -410,8 +410,8 @@ func (rc *RunContext) startJobContainer() common.Executor {
 		jobContainerNetwork = networkName
 
 		rc.JobContainer = container.NewContainer(&container.NewContainerInput{
-			Cmd:            nil,
+			Cmd:            []string{"/bin/sleep", fmt.Sprint(rc.Config.ContainerMaxLifetime.Round(time.Second).Seconds())},
-			Entrypoint:     []string{"/bin/sleep", fmt.Sprint(rc.Config.ContainerMaxLifetime.Round(time.Second).Seconds())},
+			Entrypoint:     nil,
 			WorkingDir:     ext.ToContainerPath(rc.Config.Workdir),
 			Image:          image,
 			Username:       username,