Allow reading registration token from file (#350)

When deploying the runner in a Docker Swarm setup, it can be useful to
read the potentially sensitive token from a secret instead of having to
pass it from an environment variable.

Co-authored-by: Frederik Ring <frederik.ring@gmail.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/350
Reviewed-by: Jason Song <i@wolfogre.com>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: m90 <m90@noreply.gitea.com>
Co-committed-by: m90 <m90@noreply.gitea.com>

examples/docker-compose/README.md

@@ -16,5 +16,8 @@
       - /var/run/docker.sock:/var/run/docker.sock
     environment:
       - GITEA_INSTANCE_URL=<instance url>
+      # When using Docker Secrets, it's also possible to use
+      # GITEA_RUNNER_REGISTRATION_TOKEN_FILE to pass the location.
+      # The env var takes precedence
       - GITEA_RUNNER_REGISTRATION_TOKEN=<registration token>
 ```