The Makefile exported DOCKER_HOST pointing at one private machine, so
every docker call made through make, `make lint` and `make check`
included, failed everywhere else. The line is gone: docker uses the
local daemon, or a DOCKER_HOST set in the environment.
`make build` now calls the new `script/build`, which stamps the version
and commit as the Makefile did. A VERSION set in the environment now
wins over `git describe`, not only one given as `make build VERSION=x`.
build, clean, install and docker-run are phony; install depends on
build. The vet target is removed: `script/test` runs `go vet` first.
Model: opus-5-5
