Added environment variable for run.sh to specify runner state file. (#377)

Added environment variable `RUNNER_STATE_FILE` to let users specify
where `run.sh` looks for the runner JSON file. Defaults to ``.runner``
to preserve the original behavior.

Addresses issue #368. It's not my preferred solution but it's the least invasive one I can think of.

I'm happy to make any changes you want.

I didn't see an appropriate place to reference the change in the documentation. I will add documentation wherever you think is appropriate.

To test this I did the following:
* Built an image with the `make docker` command and pushed it to my private registry.
* Added that private image as the image in my existing docker stack that was exhibiting the behavior described in #368.
* Added the RUNNER_STATE_FILE environment variable pointing to the runner JSON file set in my `config.yml` (``/data/runner.json`` in this case).
* Configured a new runner in gitea and added the token as an environment variable in the stack config.
* Deployed the stack and verified the new runner was recognized (in the idle state).
* Force updated the runner service to restart the container.
* Verified the same runner was still recognized in gitea (in the idle state) once it was back up and running.

Here is the relevant config. It's stored as a template so I've left the things that would normally be redacted as they are in the template.

```
    runner:
     image: hub.hax.in.net/haxwithaxe/act_runner:dev1
      networks:
        - swarm-net
      environment:
       RUNNER_STATE_FILE: /data/runner.json
        CONFIG_FILE: /data/config.yml
        GITEA_INSTANCE_URL: "https://git_gitea"
        GITEA_RUNNER_REGISTRATION_TOKEN: "{{ git_runner_reg_token }}"
        GITEA_RUNNER_NAME: "git_runner"
      volumes:
        - runner_data:/data
        - /var/run/docker.sock:/var/run/docker.sock
```

`runner_data` is a glusterfs subvolume

Thanks for creating this gitea specific fork. Apart from the issue this pull request addresses it works so well I don't have to think about it once it's set up.

Co-authored-by: haxwithaxe <spam@haxwithaxe.net>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/377
Co-authored-by: haxwithaxe <haxwithaxe@noreply.gitea.com>
Co-committed-by: haxwithaxe <haxwithaxe@noreply.gitea.com>

scripts/run.sh

@@ -6,6 +6,8 @@ fi
 
 cd /data
 
+RUNNER_STATE_FILE=${RUNNER_STATE_FILE:-'.runner'}
+
 CONFIG_ARG=""
 if [[ ! -z "${CONFIG_FILE}" ]]; then
   CONFIG_ARG="--config ${CONFIG_FILE}"
@@ -21,8 +23,9 @@ if [[ -z "${GITEA_RUNNER_REGISTRATION_TOKEN}" ]] && [[ -f "${GITEA_RUNNER_REGIST
 fi
 
 # Use the same ENV variable names as https://github.com/vegardit/docker-gitea-act-runner
+test -f "$RUNNER_STATE_FILE" || echo "$RUNNER_STATE_FILE is missing or not a regular file"
 
-if [[ ! -s .runner ]]; then
+if [[ ! -s "$RUNNER_STATE_FILE" ]]; then
   try=$((try + 1))
   success=0