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

.dockerignore

@@ -4,3 +4,6 @@ tmp
 .DS_Store
 *.log
 .claude
+
+# .git is sent so the build can stamp the version, without its config.
+.git/config