Use vfs storage driver for inner dockerd

overlay2 (the default) tries to mount an overlayfs layer on top of
the job container's own overlay-mounted root filesystem, which the
kernel rejects (invalid argument). vfs sidesteps nested overlay
mounts entirely.

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

.forgejo/workflows/build.yml

@@ -30,7 +30,9 @@ jobs:
             > /etc/apt/sources.list.d/docker.list
           apt-get update
           apt-get install -y --no-install-recommends docker-ce docker-ce-cli containerd.io
-          dockerd &
+          # vfs avoids nested overlayfs (the job container's own root is
+          # itself an overlay mount, and overlay-on-overlay fails).
+          dockerd --storage-driver=vfs &
           for i in $(seq 1 30); do
             docker info >/dev/null 2>&1 && break
             sleep 1