Report the version: docker build . stamps the git tag or short commit (closes #7)

attrsum --version now prints the git tag or short commit. make build
stamps it with -X from git describe, and the Dockerfile takes the VERSION
build argument when given, otherwise git describe --tags --always on the
.git in the build context, failing if .git is present and no version
comes out. A new .dockerignore, the canonical one, keeps .git/config out
of the context, and also this repo's host-built /attrsum. CI checks out
full history so it sees the 1.0.0 tag and stamps what a full clone does.
script/docker is replaced with the canonical copy.

Model: opus-5-5

Dockerfile

@@ -29,7 +29,21 @@ RUN make check
 
 # Build the binary (still as the unprivileged user: it owns /src, so git VCS
 # stamping sees consistent ownership).
-RUN make build
+#
+# The version stamped into the binary: the VERSION build argument when one
+# is given, otherwise `git describe --tags --always` of the .git the build
+# context carries: the tag on a tagged commit, tag-N-gHASH on a commit after
+# one, the short commit when no tag is reachable. A context that carries .git
+# and still yields no version fails the build. With neither, as from a source
+# tarball, the binary reports dev.
+ARG VERSION
+RUN version="${VERSION:-$(git describe --tags --always)}"; \
+    if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
+        [ "$version" = unknown ]; }; then \
+        echo "version is '$version' although .git is present" >&2; \
+        exit 1; \
+    fi; \
+    make build VERSION="${version:-dev}"
 
 # Runtime stage
 # alpine 3.21, 2026-02-28