On Mac OS X `readlink -f` is not compatible in gke_bootstrap_script.sh

On Mac OS X, the readlink -f command has a different (BSD) not compatible behavior and when running

gke_bootstrap_script.sh
...
readlink: illegal option -- f
usage: readlink [-n] [file ...]

I looked up the man pages for readlink on Mac OS X and Debian and the -f option has a different, non-compatible behavior.

I did not find a suitable fix to make a Merge Request.

On my Mac, I was able as a work-around to use:

DIR=$(dirname "$(readlink "$0")")