Moves the Containerfile and build.sh added in the previous CI fix up
into bread-ecosystem/ci/, so other GTK4 layer-shell products in the
ecosystem can share it instead of each carrying their own copy. This
repo's ci/build.sh is now a thin wrapper that clones bread-ecosystem
at the commit pinned in ci/bread-ecosystem.rev and delegates to its
ci/build.sh — pinned by sha rather than tracking main, so an unrelated
change over there can't silently break this repo's release builds
(the failure mode the bread-theme test fix a moment ago was caused by).
Verified locally: clippy, test, and release build all pass through
the new delegator path.
