Avoid warning about running Bundler as root

[elia]

fixes #10

Shouldn't be a problem for earlier versions of bundler

[yosifkit]

This seems like a good change to me. 👍

My only thought is why not just add it to the ruby image so that all ruby users benefit? Probably just add another line to the RUN in each of the versions.

[elia]

My only thought is why not just add it to the ruby image so that all ruby users benefit?

I added it along with another bundle config command, but I agree it would be a good addition.