Run-time composition instead of baked-in toolchain combinations: a slim
base image plus per-toolchain prefix binds (--with jdk-21 --with go-1.24),
possible because official toolchain tarballs live under their own
prefixes — plain ro-binds, no overlayfs, no root, today's bwrap.
Comes due when a second toolchain combination is needed; until then the
one fat image stays the deliberate choice.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
