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` without
`.git/config`, which can hold a credential, 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 a non-empty `VERSION`
from the command line or the environment, 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`, directory or file, and it comes
out empty, `dev` or `unknown`.

Model: opus-5-5

script/docker

@@ -14,8 +14,8 @@ main() {
     cd "$ROOT"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
-    # excludes .git, so `git describe` in a build stage cannot find it.
+    # empty constant. The VERSION build arg takes precedence over what
+    # the build would derive from the .git in its context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache-filter=lint,builder \