Fix build of debian images failing on "gem update --system"

[rata]

If we run it later, it fails with:

    gem update --system 2.6.13
    ERROR:  Loading command: update (LoadError)
            cannot load such file -- zlib
    ERROR:  While executing gem ... (NoMethodError)
        undefined method `invoke_with_build_args' for nil:NilClass

[tianon]

Not sure I understand the issue -- can you please elaborate?

https://travis-ci.org/docker-library/ruby/builds/278778774 is passing (the most recent Travis build of all versions/tags), and the Docker image is successfully building on the official images build servers 😕

[rata]

@tianon it fails to build here with that error, the stretch slim at least. It fails here because the zlib packages are removed just before doing the "gem update".

Not sure why travis fails to build the slim image this commits fixes on my docker. I'll revisit that.

Thanks!

[rata]

@tianon oh, I see it now. It was my bad, sorry.

I had a typo here, using a different "FROM". It's a debian:stretch but with some added packages, and the purge makes it fail. If I use "FROM debían:stretch" (just as it is here), it works fine.

I'm really sorry for the confusion. And sorry for this, I hope my next contribution is more helpful :)