Stamp the build version into the binary (closes #253)

script/version

@@ -0,0 +1,65 @@
+#!/bin/sh
+# script/version: output the version string the binary is stamped with.
+# Our own extension to scripts-to-rule-them-all. The Makefile's build
+# target and script/docker both take the value from here, so a `make
+# build` binary and a `make docker` image built from the same checkout
+# report the same thing.
+#
+# Order of precedence:
+#
+#   1. $VERSION, if set and non-empty. This is how the value reaches a
+#      build that cannot derive it: .dockerignore excludes .git/, so the
+#      builder stage has no git metadata and the Dockerfile takes the
+#      value as a build arg instead.
+#   2. `git describe --tags --always --dirty` against this checkout. At
+#      a clean tagged commit that is exactly the tag; otherwise it
+#      carries the short SHA, the commit distance when a tag is
+#      reachable, and a -dirty suffix for uncommitted changes.
+#   3. "unknown", for a tree with no git metadata and no $VERSION -- a
+#      source tarball, or `docker build .` with no --build-arg. That
+#      case must not fail the build and must not name a tag the tree may
+#      not be at, so it names nothing.
+#
+# The git step insists the enclosing repository is this checkout, not
+# merely some repository above it: an unpacked tarball sitting inside an
+# unrelated working copy would otherwise be stamped with that copy's
+# version.
+#
+# Nothing here may vary between two builds of the same commit: the
+# release gate asserts the binary is byte-identical across builds. That
+# rules out a build timestamp, a hostname, and a builder identity.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+# in_this_checkout succeeds when git can read metadata for a repository
+# whose work tree root is $ROOT.
+in_this_checkout() {
+    command -v git >/dev/null 2>&1 || return 1
+
+    top="$(git rev-parse --show-toplevel 2>/dev/null)" || return 1
+    [ -n "$top" ] || return 1
+
+    top="$(cd "$top" 2>/dev/null && pwd -P)" || return 1
+    [ "$top" = "$ROOT" ]
+}
+
+main() {
+    if [ -n "${VERSION:-}" ]; then
+        echo "$VERSION"
+
+        return 0
+    fi
+
+    cd "$ROOT"
+
+    if in_this_checkout; then
+        # --always keeps an untagged history from failing the build: it
+        # falls back to the bare short SHA.
+        git describe --tags --always --dirty 2>/dev/null && return 0
+    fi
+
+    echo "unknown"
+}
+
+main "$@"