Keep .git/config out of the Docker build context (closes #269)

.git goes into the build so `make build` can stamp the version, and with
it went .git/config, where a remote URL can carry a credential that then
stays in the builder stage's layers on the build host. `git describe`
does not need it, so .dockerignore now leaves it out.

Model: opus-5-5

TODO.md

@@ -20,6 +20,10 @@ regress.
 
 # Completed Steps
 
+- 2026-10-02: `.dockerignore` leaves out `.git/config`, so a remote URL there
+  that carries a credential no longer goes into the Docker build; the image
+  still shows the commit it was built from (#269).
+
 - 2026-10-01: Built images are tagged `upaas-<app>:<short hash>`, git's short
   form of the commit built, instead of the deployment number. A redeploy of a
   commit gives its tag to the new image; the old one is kept while the app runs