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