Fix commit hash not showing in Docker builds (#43)

* Fix commit hash not showing in Docker builds

build_info.js computed buildHash via `git rev-parse --short HEAD` at
runtime, but the final image intentionally has no git binary and no
.git directory (kept lean, per .dockerignore) — so this always failed
silently and the footer's version line showed "unknown" for every
Docker deployment. Working correctly only for bare-metal/dev, where
git + .git are actually present.

Added a throwaway gitinfo build stage that reuses the main base image
(no extra pull) with git installed just for this stage, reads .git
from the build context (now no longer excluded — see .dockerignore),
and bakes the resolved short hash into a small file that IS copied
into the final image. build_info.js reads that file first, falling
back to the old git-rev-parse behavior (still needed for bare-metal).

Verified against a real build: `docker exec sso-manager cat
/app/.build_commit` matches `git rev-parse --short HEAD` on the host,
and the footer now shows the real hash instead of "unknown". Same fix
already applied to proxy (theta42/proxy#133).

* Support GIT_COMMIT build-arg override for submodule builds

The gitinfo stage from the previous commit works for a standalone
clone (.git is a real directory) but not when this repo is built as a
git submodule (e.g. from theta-env): a submodule's .git is a pointer
FILE, not a directory — the real object database lives in the
superproject's .git/modules/, outside this repo's own directory and
therefore outside Docker's build context entirely. `git rev-parse`
can never resolve it from in here no matter what, so builds via
theta-env still baked in "unknown" despite the earlier fix.

Add an optional GIT_COMMIT build-arg that, when set, wins over the
in-context git resolution. theta-env's setup.sh now computes it on the
host (where the submodule DOES resolve correctly) and passes it via
docker-compose.yml's build.args. Same fix in proxy: theta42/proxy#133.

Verified via theta-env's actual setup.sh end to end: rebuilding with
this change, `docker exec sso-manager cat /app/.build_commit` now
matches `git -C sso-manager-node rev-parse --short HEAD` on the host
(previously: "unknown").

.dockerignore

@@ -1,5 +1,8 @@
 # Git
-.git
+# NOTE: .git is intentionally NOT excluded — the gitinfo build stage in
+# Dockerfile.openldap reads it to bake the commit hash into the image (see
+# nodejs/utils/build_info.js), then it's discarded before the final stage.
+# It never ends up in the final image.
 .gitignore
 
 # Development