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

.dockerignore

@@ -1,9 +1,9 @@
-.git/
+# .git is sent, without its config: the builder stage derives the version it
+# stamps into the binary from it, and `git describe` does not need the config,
+# which can hold a credential (a password in the remote URL, a CI token). No
+# tracked file may be listed here: git in the build would see it as deleted
+# and mark the version -dirty, and an excluded .md would silently drop out of
+# the prettier check in Dockerfile.fmt.
+.git/config
 bin/
 node_modules/
-# No .md may be excluded: Dockerfile.fmt checks every document with
-# prettier, and an exclusion here would drop a file from that check while
-# prettier still reports every file it was handed clean.
-LICENSE
-.editorconfig
-.gitignore