Fix webpack git access in Docker build

- Copy .git/modules/jellyfin-web to Docker image so webpack can run git describe
- Add SSH agent forwarding (-A) to build_truenas.sh for git submodule cloning

docker/Dockerfile

@@ -63,6 +63,8 @@ RUN apk add \
  && git config --global --add safe.directory /jellyfin/jellyfin-web
 
 WORKDIR ${SOURCE_DIR}
+# Copy submodule git metadata to parent directory where jellyfin-web/.git references it
+COPY .git/modules/jellyfin-web /.git/modules/jellyfin-web
 COPY jellyfin-web .
 
 RUN npm ci --no-audit --unsafe-perm \