Support --ephemeral option

[Kesin11]

fix: #148

Add EPHEMERAL environment variable to support --ephemeral option.

@joeyparrish refers also need to fix ephemeral_runner.sh, but it seems no problem with using default ENTRYPOINT and CMD.
I checked using this docker run command.

docker run --rm --name github-runner \
  -e RUNNER_NAME_PREFIX="ephemeral-docker" \
  -e ACCESS_TOKEN="${RUNNER_PAT}" \
  -e RUNNER_SCOPE="org" \
  -e ORG_NAME="${ORG}" \
  -e LABELS="container" \
  -e EPHEMERAL=1 \
  -v /var/run/docker.sock:/var/run/docker.sock \
  kesin11/docker-github-actions-runner:latest

[myoung34]

Closes #148

[myoung34]

Thanks!

[Kesin11]

Thanks all for the fast response and merge!