docker: a plain docker build . stamps the git version (closes #210)

A plain `docker build .`, which is how upaas builds, stamped `dev`:
`.dockerignore` left out `.git` and the builder declared
`ARG VERSION=dev`. `.dockerignore` now sends `.git` and lists no tracked
file, which git in the build would count as deleted and mark `-dirty`.
`ARG VERSION` has no default. The Makefile takes `VERSION` from the
environment with `?=`, so a build arg still wins (`script/docker` keeps
passing one); otherwise `git describe` runs in the builder. A new
`make version` prints the version, and the builder fails when the
context carries `.git` and it comes out empty, `dev` or `unknown`.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-02: a plain `docker build .` of a clone stamps its tag or short
+  commit, not `dev`: the build context now carries `.git` (closes #210).
 - 2026-10-01: a certificate within the expiry warning period is warned about on
   every TLS check, where some checks used to skip it at random (closes #204).
 - 2026-10-01: a domain's NS set is its delegation from the parent zone's