Stamp the git tag or short commit into the binary (closes #4)

A plain `docker build .` now stamps the version into bsdaily: the
VERSION build argument when one is given, otherwise `git describe
--tags --always` of the .git in the build context. The build fails if
the context carries .git and no version comes out. A host `make` build
stamps the same `git describe` value, or dev when it yields nothing.
bsdaily logs the version on the first line of every run and prints it
with --version.

The new .dockerignore is the canonical copy, which keeps .git/config
out of the build context, plus this repo's host-built artifacts.
script/docker is replaced with the canonical copy, which passes the
version it derives on the host.

Model: opus-5-5

cmd/bsdaily/main.go

@@ -10,6 +10,11 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// Version is the git tag or short commit, set at link time with -X by the
+// Dockerfile and the Makefile, logged on the first line of every run and
+// printed by --version. Builds that do not set it report dev.
+var Version = "dev"
+
 func main() {
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
 		Level: slog.LevelInfo,
@@ -23,8 +28,11 @@ func main() {
 	rootCmd := &cobra.Command{
 		Use:          "bsdaily",
 		Short:        "Extract a single day's data from the latest daily snapshot",
+		Version:      Version,
 		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {
+			slog.Info("starting", "version", Version)
+
 			hasDate := dateFlag != ""
 			hasFrom := fromFlag != ""
 			hasTo := toFlag != ""