fix: show git commit in About instead of "unknown"

Dockerfile

@@ -8,6 +8,10 @@ ENV NEXT_TELEMETRY_DISABLED=1
 # at build time, so it cannot be changed without rebuilding.
 ARG NEXT_PUBLIC_BASE_PATH=
 ENV NEXT_PUBLIC_BASE_PATH=$NEXT_PUBLIC_BASE_PATH
+# Commit SHA shown in the About screen. .dockerignore excludes .git, so
+# `git rev-parse` inside the build can't find it — CI must pass it in.
+ARG GIT_COMMIT=unknown
+ENV GIT_COMMIT=$GIT_COMMIT
 RUN npx next build --webpack
 
 FROM node:24-alpine AS runner