script/build stamps lightweight tags too (closes #156)

When VERSION is unset or empty, script/build now takes the version from
`git describe --tags --always --dirty`, the form script/docker and
script/cibuild use. Without --tags it ignored lightweight tags, so a
binary from `make build` could report a commit where the image built
from the same tree reported the tag. README.md Entrypoints names the
same form.

Model: opus-5-5

README.md

@@ -55,7 +55,8 @@ either one is broken. We provide:
 - `script/build` — build the frontend for production into `dist/`, then the
   static `netwatch-server` binary into the repo root with its version stamped
   in: `VERSION` from the environment, or when that is unset or empty,
-  `git describe` inside a git checkout, then `dev`
+  `git describe --tags --always --dirty` inside a git checkout, as
+  `script/docker` does, then `dev`
 - `script/run` — build with `script/build`, then run `netwatch-server` with its
   defaults (see [Settings](#settings))
 - `script/clean` — remove what `script/build` writes