Put step-runner binary in PATH in container

Over in gitlab-runner!5069 (merged) we mention that step-runner must be in $PATH for runner steps integration to work. This is actually not true at the moment because runner explicitly invokes step-runner at /. To make that statement true and keep everything else running as is, I've added hard-link from /step-runner to /usr/bin/step-runner. It works:

    > docker run -it --rm --entrypoint step-runner step-runner -h
    Step Runner executes a series of CI steps

    Usage:
    step-runner [command]

    Available Commands:
    ci          Run steps in a CI environment variable STEPS
    completion  Generate the autocompletion script for the specified shell
    help        Help about any command
    proxy       Tunnel gRPC requests/responses from stdin/stdout to the service listening on a local socket
    run         Run a step locally
    serve       Start the step-runner gRPC service

    Flags:
    -h, --help   help for step-runner

    Use "step-runner [command] --help" for more information about a command.