implement act_runner rootless image (#208)

This PR creates a rootless Docker image that runs both `dockerd` and `act_runner` using `supervisord`.  It has been tested locally for a few days and seems stable.

Co-authored-by: ccureau <[email protected]>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/208
Reviewed-by: Jason Song <[email protected]>
Co-authored-by: ccureau <[email protected]>
Co-committed-by: ccureau <[email protected]>

scripts/supervisord.conf

@@ -0,0 +1,13 @@
+[supervisord]
+nodaemon=true
+logfile=/dev/null
+logfile_maxbytes=0
+
+[program:dockerd]
+command=/usr/local/bin/dockerd-entrypoint.sh
+
+[program:act_runner]
+stdout_logfile=/dev/fd/1
+stdout_logfile_maxbytes=0
+redirect_stderr=true
+command=/opt/act/rootless.sh