Change thumbnail mounting dir to avoid nested mounts

[Rivers47]

Please describe your suggestion, and the problem it'd solve. Currently the docker image mounts thumbnail to content/thumb which is very convoluted and LRRG will try to create a empty directory "thumb/ under content if user creates a thumbnail volume that mounts to content/thumb. Mounting a volume inside another bind mount should be avoided. I suggest either in tools/build/docker/Dockerfile remove

VOLUME [ "/home/koyomi/lanraragi/content/thumb" ]

alltogether and let user decide where thumbnails should be, or change it to

VOLUME [ "/home/koyomi/lanraragi/thumb" ]

Additional context none