[CI] Test building for 32-bit arch

[hcho3]

Closes #8964
Closes #10015

• Attempt a minimal change to code base to eliminate narrowing errors from the compiler.
• Also experiment with docker/build-push-action, which massively simplifies caching of Docker containers. It transparently uses GitHub's built-in caching feature to cache containers.
• The 32-bit Debian container ships with latest GCC (13) which throws warning for overloaded virtual functions. For now, this warning would not be considered an error.

[trivialfis]

Thank you for working on the 32-bit build!

We will probably need to have some follow-up PRs for running some basic tests in C++, but there is no need to rush. Feel free to merge as you see fit.

[barracuda156]

@hcho3 Thank you very much for working on this!

@trivialfis Will this be included into the next CRAN release? (I recall I could not simply apply patches from the master onto R package code.)

[hcho3]

Will this be included into the next CRAN release?

Probably not, since we are already having trouble uploading XGBoost 2.0 to CRAN. We are currently discussing next steps, see #9810.
In the meanwhile, you are free to build XGBoost R package from the source. (Use R CMD INSTALL . from the R-package directory.)

[barracuda156]

@hcho3 Thanks, then I can set it up manually with cloning the repo to run tests. (I need Macports environment for R stuff to work correctly.) Will update on results.