Cannot build/test older (e.g. < 5.20 Perl images) on modern buildpack-deps

[zakame]

As the images use buildpack-deps:latest as their base, some versions of Perl can no longer properly build or test, due to the update churn of the build toolchain. I encountered this now in part of investigating #17 and whether it still occurs in newer Docker releases.

I think we can start checking on using tagged buildpack-deps per Perl version and adjust appropriately.