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

script/docker

@@ -1,6 +1,8 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
+# --no-cache because the gate phases the final stage depends on are RUN
+# steps, and a cached one is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +10,15 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"