Mount a read-only folder inside a Docker container with OverlayFS on top

overlay.sh

# On the host to run the container
docker run --privileged -i -t -v ~/host-folder-to-mount:/root/folder-ro:ro ubuntu

# Inside the container
# Need to create the upper and work dirs inside a tmpfs.
# Otherwise OverlayFS complains about AUFS folders. 
mkdir -p /tmp/overlay && \
mount -t tmpfs tmpfs /tmp/overlay && \
mkdir -p /tmp/overlay/{upper,work} && \
mkdir -p /root/folder && \
mount -t overlay overlay -o lowerdir=/root/folder-ro,upperdir=/tmp/overlay/upper,workdir=/tmp/overlay/work /root/folder