feat: allow ephemeral runner to be optional (#498)

- Adds `ephemeral` option to `runner.spec` 
    
    ```
      ....
      template:
         spec:
             ephemeral: false
             repository: mumoshu/actions-runner-controller-ci
      ....
    ```
- `ephemeral` defaults to `true`
- `entrypoint.sh` in runner/Dockerfile modified to read `RUNNER_EPHEMERAL` flag
- Runner images are backward-compatible. `--once` is omitted only when the new envvar `RUNNER_EPHEMERAL` is explicitly set to `false`.

Resolves #457

runner/entrypoint.sh

@@ -97,5 +97,10 @@ for f in runsvc.sh RunnerService.js; do
   sudo mv {patched,bin}/${f}
 done
 
+args=()
+if [ "${RUNNER_EPHEMERAL}" != "false" ]; then
+  args+=(--once)
+fi
+
 unset RUNNER_NAME RUNNER_REPO RUNNER_TOKEN
-exec ./bin/runsvc.sh --once
+exec ./bin/runsvc.sh "${args[@]}"