Add --userns flag to support Docker daemon that enables user namespace (#533)

I got an error like this after hitting `act` command.

> Error: Error response from daemon: cannot share the host's network namespace when user namespaces are enabled

According to the document, when user namespaces are enabled on the Docker daemon,
neither host network mode and --privileged work without --userns=host. Since `act`
uses host network mode to match GitHub Actions runners, it cannot run jobs when
user namespaces are enabled. So I added the flag.

https://docs.docker.com/engine/security/userns-remap/#user-namespace-known-limitations

Co-authored-by: Casey Lee <cplee@nektos.com>

README.md

@@ -105,6 +105,7 @@ It will save that information to `~/.actrc`, please refer to [Configuration](#co
   -r, --reuse                  reuse action containers to maintain state
   -s, --secret stringArray     secret to make available to actions with optional value (e.g. -s mysecret=foo or -s mysecret)
       --secret-file string     file with list of secrets to read from (e.g. --secret-file .secrets) (default ".secrets")
+      --userns string          user namespace to use
   -v, --verbose                verbose output
       --version                version for act
   -w, --watch                  watch the contents of the local repo and run when files change