Add 2.5-rc builds

[mtsmfm]

Please merge docker-library/official-images#3606 first 🙏
Current test suite is failed because of removing mathn.rb from stdlib since Ruby 2.5

---

Ruby 2.5.0-preview1 is released.

https://www.ruby-lang.org/en/news/2017/10/10/ruby-2-5-0-preview1-released/

I ran following commands to create them:

cp -r 2.4 2.5-rc
./update.sh

[mtsmfm]

Now CI is passed 💚

[tianon]

We should ditch jessie and alpine3.4 variants for 2.5+. 👍

Also, I'm not so sure about adding --force to the bundler install -- bundler was not included in previous versions and now is, or it's now at the same version as the latest so fails to install? (We've had lots of issues around this sort of thing with Python and PIP, so my default reaction to these types of problems is now being wary. 😅)

If bundler was not included and is now, I wonder if perhaps for 2.5+ we should stop installing our own version and just let whatever comes with Ruby stay (especially since as written, our final image will end up with two copies of bundler since the reinstall happens in a later layer).

[mtsmfm]

Bundler is default gem since Ruby 2.5.

https://www.ruby-lang.org/en/news/2017/10/10/ruby-2-5-0-preview1-released/

I add --force option to avoid following message:

bundler's executable "bundle" conflicts with /usr/local/bin/bundle
Overwrite the executable? [yN]

If bundler was not included and is now, I wonder if perhaps for 2.5+ we should stop installing our own version and just let whatever comes with Ruby stay

I think we should do as same as rubygems.
We already have updated rubygems.

ruby/Dockerfile-debian.template

Line 60 in 59abfdf

&& gem update --system "$RUBYGEMS_VERSION"

So specifying bundler version and re-installing makes sense to me.

especially since as written, our final image will end up with two copies of bundler since the reinstall happens in a later layer

So then, what do you think about inlining?

...
ENV BUNDLER_VERSION %%BUNDLER%%
...
&& rm -r /usr/src/ruby \
	\
	&& gem update --system "$RUBYGEMS_VERSION" \
        && gem install bundler --version "$BUNDLER_VERSION" --force
...

[tianon]

Ok, fair enough -- moving that inline like our "rubygems" force update makes sense then. Thanks!

[mtsmfm]

@tianon Updated!
Could you review again?

[tianon]

Thanks!