CI: migrate to bread-ecosystem's shared Arch build image

Routes the release build/test steps and a new fast pre-release
check.yml through ci/build.sh, which delegates to bread-ecosystem's
pinned shared Containerfile/build.sh instead of building against
whatever the bare hestia runner happens to have installed. Follows
the pattern already proven in breadpad. No extra ci/deps.txt: bread's
system_deps (systemd-libs, openssl, zlib) are all already pulled in
transitively by the shared image's base + git packages.

ci/build.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Delegates to bread-ecosystem's shared CI build image/script, pinned to
+# the commit in ci/bread-ecosystem.rev — not `main`. bread-ecosystem's CI
+# files now affect every product's release pipeline, so bumping the pin
+# is a deliberate act instead of silent drift (see the bread-theme test
+# that broke here for exactly that reason, before it was pinned by rev).
+#
+# Usage: ci/build.sh cargo build --release --locked
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+REV="$(cat "${ROOT}/ci/bread-ecosystem.rev")"
+
+CACHE_DIR="/tmp/bread-ecosystem-ci-${REV}"
+if [ ! -d "$CACHE_DIR" ]; then
+    rm -rf /tmp/bread-ecosystem-ci-*
+    git clone https://git.breadway.dev/Breadway/bread-ecosystem.git "$CACHE_DIR"
+    git -C "$CACHE_DIR" checkout --quiet "$REV"
+fi
+
+bash "${CACHE_DIR}/ci/build.sh" bread "$ROOT" "$@"