Make centos and gnu thin, aligning with musl

[lloeki]

The targeted global generalisable pattern is:

• engines/<engine>:<version>: thin image, just the runtime; use this if you don't care what the underlying libc/OS userland is and just want a Ruby runtime at said version; today this is gnu-backed but could transparently become musl-backed in the future.
• engines/<engine>:<version>-<libc>: thin image, just the runtime; use this if you care what the underlying libc is and just want a Ruby runtime at said version and on said libc.
• engines/<engine>:<version>-<libc>-<compiler>: fat image, runtime+compiler+whatever necessary to build extensions; use this if you care what the underlying libc is and want a Ruby buildtime environment at said version and on said libc and with said compiler.

The goal is to be able to:

• Reliably produce minimal images via multistage builds (e.g depend on FROM -gnu-gcc on a bundle install intermediate stage, then copy over /usr/loca/bundle to a final stage that FROMs just -gnu)
• Be explicit about what a particular image consumer actually depends on
• Maximise shared layers between images while minimising individual layer and image size

Since various repos and tooling use ruby:2.1 to ruby:3.2 and jruby-9.2 to jruby-9.4 and expect a compiler (i.e as if they were *-gnu-gcc) it was elected to have these "lie" and be aliases to -gnu-gcc for backwards compatibility.

This results in:

• ruby:2.1-gnu-gcc to ruby:3.2-gnu-gcc, jruby-9.2-gnu-gcc to jruby-9.4-gnu-gcc
• ruby:2.1-gnu to ruby:3.2-gnu, jruby-9.2-gnu to jruby-9.4-gnu, alias tags of the above
• ruby:2.1 to ruby:3.2, jruby-9.2 to jruby-9.4, alias tags of the above
• ruby:2.1-musl to ruby:3.2-musl, jruby-9.2-musl to jruby-9.4-musl, thin alpine-based images without compiler
• ruby:2.1-musl-gcc to ruby:3.2-musl-gcc, jruby-9.2-musl-gcc to jruby-9.4-musl-gcc, based off of the above (FROM), with gcc

engines/<engine>:<version>-centos and engines/<engine>:<version>-centos-gcc follow the same aliasing as for gnu.

Task is to make:

• engines/<engine>:<version>-gnu thin
• engines/<engine>:<version>-gnu-gcc fat
• engines/<engine>:<version>-centos thin
• engines/<engine>:<version>-centos-gcc fat

Requires: #26