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 would count as deleted. `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; otherwise `git describe` runs in
the builder, which trusts the checkout whoever owns it, as a context
sent as a tar archive keeps its owners. A new `make version` prints the
version; the build fails when the context carries `.git` 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 \