Support basic, dind and dind-rootless as multiple kinds of images (#619)

- `basic`: Only the runner process in the container; users need to mount the Docker socket to it.
- `dind`: A Docker daemon will be started in the container with the root user.
- `dind-rootless`: A Docker daemon will be started in the container with a rootless user.

Use s6 instead of supervisord to start processes.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/619
Reviewed-by: Zettat123 <[email protected]>
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: Jason Song <[email protected]>
Co-committed-by: Jason Song <[email protected]>

scripts/s6/docker/finish

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+exec s6-svscanctl -t /etc/s6
+

scripts/s6/docker/run

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+exec dockerd-entrypoint.sh