Stamp the git tag or short commit in a plain docker build (closes #67)

.dockerignore now sends .git, without .git/config, which can hold a
credential. The build stage takes the VERSION build argument when one
is given, otherwise git describe --tags --always of that .git, and
fails if the context carries .git and still yields no version. A plain
docker build . used to stamp dev. The CI checkout fetches full history
so CI sees the tag and stamps the same value as make build.

Model: opus-5-5

.dockerignore

@@ -1,4 +1,9 @@
-.git
+# .git is sent without its config. Without a VERSION build argument the
+# stage that compiles runs `git describe --tags --always` on .git, which
+# does not need .git/config; that file can hold a credential, such as a
+# password in a remote URL or the token the CI checkout step stores there.
+.git/config
+
 .claude
 .DS_Store
 sfdupes