Improve build progress and repeatability

[gauravtiwari]

What changed

• move Docker-style progress and live command output onto Dagger Ruby 0.11
• show build, runtime, and export steps without Dagger startup noise
• make interruption clean and keep engine failures visible
• use available CPUs for Bundler unless configured otherwise
• keep Rails artifacts identical across cold and restored dependency caches

Verification

• bin/ci on Ruby 3.2.11 and Ruby 4.0.6
• live Docker Dagger build with BoringCache restore
• repeat Rails artifacts are byte-identical
• live Ctrl+C exits 130 without traceback, orphan, or zombie processes