Send .git without its config; correct shallow-clone and version comments

.dockerignore lists .git/config, which holds the clone's remote URL and
any credential in it; the build stage is the final image, so it would
otherwise ship. git describe does not need it. The build-context test
asserts the entry, and the README says the image carries .git without
its config.

The README now says a shallow clone stamps a tag only when the cloned
commit itself carries one, and otherwise the short commit. The comment in
src/index.ts says a build reports the version script/build stamps into
dist/package.json, and package.json's own version only from source.

Model: opus-5-5

.dockerignore

@@ -3,7 +3,10 @@
 # and dropping it would change what the lint phase's prettier check sees.
 #
 # .git is deliberately NOT excluded: the build derives the version it stamps
-# from it (script/version).
+# from it (script/version). It is sent without its config, which holds the
+# clone's remote URL and any credential in it, and which the build stage, the
+# final image, would otherwise carry. git describe does not need it.
+.git/config
 
 # OS
 .DS_Store