build: release a static image with scripts/release.sh

The image now packages a cross-compiled musl binary on distroless instead of
compiling inside Docker. The script builds it, pushes :<version> and :latest
to git.cosocial.info, and tags the commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -1,5 +1,4 @@
-target
-.git
-.env
-*.sqlite
-*.sqlite-*
+# only the release binary and the city list go into the image
+*
+!target/x86_64-unknown-linux-musl/release/cities-labeler
+!data