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

Dockerfile

@@ -36,11 +36,25 @@ COPY . .
 # Run the tests - build fails if any test fails
 RUN make test
 
-# Build the binary. .dockerignore leaves out .git, so `git describe` in
-# the Makefile cannot find the version here: script/docker passes it as
-# --build-arg VERSION, and a build that passes none reports `dev`.
-ARG VERSION=dev
-RUN make build VERSION="${VERSION}"
+# Version stamped into the binary: the VERSION build arg when one is
+# given (script/docker passes one), otherwise what `git describe` says of
+# the .git in the build context, so a plain `docker build .` of a clone
+# stamps its tag or short commit. The build arg reaches make through the
+# environment.
+ARG VERSION
+
+# A context that carries .git must yield a real version: one that is
+# empty, `dev` or `unknown` cannot be traced back to a commit.
+RUN version="$(make version)"; \
+    if [ -d .git ]; then \
+        case "$version" in \
+            "" | dev | unknown) \
+                echo "version is \"$version\" although the build context carries .git" >&2; \
+                exit 1 ;; \
+        esac; \
+    fi
+
+RUN make build
 
 # Runtime stage
 # alpine 3.21, 2026-02-28