Stamp the git tag or short commit in a plain docker build (closes #86)

The builder stage now has git and takes the version from the VERSION
build argument when one is given, otherwise from `git describe --tags
--always` of the repo's .git, copied to /git so go build does not see
it. A version that still comes out empty, dev or unknown fails the
build. ARG VERSION has no default. .dockerignore keeps .git/config out
of the build context, and the comments in script/docker and
script/cibuild no longer say .git is excluded.

Model: opus-5-5

script/cibuild

@@ -16,9 +16,8 @@ main() {
     "$SCRIPT_DIR/check"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
-    # excludes .git, so `git describe` in a build stage yields an empty
-    # version without failing.
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \