Fix build version showing "unknown" in production

The previous approach baked git rev-parse --short HEAD into the image
at Docker build time, but Dockhand's build context doesn't reliably
have .git available, so it silently fell back to "unknown" in
production even though the build itself succeeded. Replace it with a
plain VERSION file committed to the repo (a timestamp + short
description, updated by hand alongside each commit) that ships via the
same COPY . . as everything else - no git access needed inside the
build at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.dockerignore

@@ -1,8 +1,5 @@
 # Version control
-# .git is intentionally NOT excluded - the Dockerfile reads it at build
-# time (git rev-parse) to bake the running commit into the image as
-# /app/BUILD_VERSION, then deletes .git itself in that same build step so
-# it never ends up in the final image.
+.git
 .gitignore
 .github