Use docker’s stdcopy to ensure we don’t emit garbage bytes to stdout

[aidansteele]

As per the ContainerAttach() docs, a container not using a TTY has stderr and stdout multiplexed into a single stream in a Docker-specific format.

When running act and its stdout is not a terminal (e.g. when it's redirected to a file), DockerExecutorInput.logDockerOutput() is invoked -- I've changed this to use github.com/docker/docker/pkg/stdcopy.StdCopy as per Docker's suggestion.

Right now the issue is the tests pass when output is redirected to a file and fail when run directly in a terminal! ❗️ This is because of the following two conflicting pieces:

act/container/docker_run.go

Lines 89 to 97 in f2cb9e3

	isTerminal := terminal.IsTerminal(int(os.Stdout.Fd()))
	config := &container.Config{
	Image: input.Image,
	Cmd: input.Cmd,
	Entrypoint: input.Entrypoint,
	WorkingDir: input.WorkingDir,
	Env: input.Env,
	Tty: isTerminal,

act/container/docker_run.go

Lines 170 to 171 in f2cb9e3

	isTerminal := terminal.IsTerminal(int(os.Stdout.Fd()))
	if !isTerminal || os.Getenv("NORAW") != "" {

Honestly, I'm not sure what the purpose of the NORAW env var is. If NORAW is passed in, the tests will fail in even more spectacular ways. I would like to do the following, but wanted to run it by you first before submitting this PR:

• Instead of terminal.IsTerminal() in attachContainer(), we use the Docker API client to introspect the container and see if it is in TTY mode or not.
• Additionally, add Tty *bool to NewDockerRunExecutorInput and only fall back to terminal.IsTerminal() if it is nil.

What are your thoughts?