CI: build on bread-ecosystem's shared Arch container

Ports breadbox onto the shared, pinned CI build image (following the
pattern proven in breadpad): ci/build.sh delegates to bread-ecosystem's
own ci/build.sh, pinned by commit sha in ci/bread-ecosystem.rev, instead
of building directly on the bare self-hosted runner. Also adds
check.yml to run clippy/test on feature/fix branches ahead of the
dev-track release build.

No ci/deps.txt: breadbox's Cargo.toml pulls in nothing beyond what the
shared Containerfile already installs (gtk4, gtk4-layer-shell). The
librsvg system dep in bakery.toml is a runtime gdk-pixbuf loader for
SVG icons, not a Cargo build dependency.

.forgejo/workflows/dev-release.yml

@@ -19,7 +19,7 @@ jobs:
             "https://git.breadway.dev/${GITHUB_REPOSITORY}.git" src
 
       - name: build
-        run: cd src && cargo build --release --locked
+        run: cd src && bash ci/build.sh cargo build --release --locked
 
       - name: compute dev version
         run: |