`serviceCredentialsFile` not passed to docker image if it points ouside of current working directory

[mateuszkwiecinski]

Originally reported in: https://github.com/wzieba/Firebase-Distribution-Github-Action/issues/52. The path passed as serviceCredentialsFile doesn't get passed to docker image hence gets ignored when calling Firebase CLI.

I expect a serviceCredentialsFile to be accessible when calling Firebase or (if that's not feasible) at least have a mention about such restriction in action's Readme file