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

attrsum.go

@@ -36,6 +36,10 @@ const (
 	progressThrottle = 250 * time.Millisecond
 )
 
+// Version is the git tag or short commit, set at link time with -X by
+// `make build`. A build that does not set it reports dev.
+var Version = "dev" //nolint:gochecknoglobals // set at link time with -X
+
 // Sentinel errors returned by the command implementations.
 var (
 	errNoPaths           = errors.New("no paths provided")
@@ -101,8 +105,9 @@ func main() {
 	opts := &options{}
 
 	rootCmd := &cobra.Command{
-		Use:   "attrsum",
-		Short: "Compute and verify file checksums via xattrs",
+		Use:     "attrsum",
+		Short:   "Compute and verify file checksums via xattrs",
+		Version: Version,
 	}
 	rootCmd.SilenceUsage = true
 	rootCmd.SilenceErrors = true