Add support for buildpack caching

[mboersma]

Having 2 optional positional args is a bit inelegant I suppose, but you can pass a cache_dir on the command line and ignore tags by doing:
./buildstep name "" /opt/caching

[elia]

Adding caching will also probably help avoiding re-downloading ruby binaries at each deploy

[jschneiderhan]

This has been woking pretty well for me. Thanks!

[bzz]

Can you point plz how to add this buildpack caching to dokku?

[progrium]

Any updates on thinking around this? Is this still the best way to go about this?

[mjonuschat]

In keeping with the Docker philosophy I would suggest a cache/data volume based on the app name. Option parsing for the script could be switched to getopts, this should make it possible to keep the old calling syntax for the time being while making new options named instead of positional.

[progrium]

Caching is supported by the builder script. This buildstep script is on its way to being deprecated.