"Processes inside Docker no longer run as root. Existing installations will require you to change directory ownership of the uploads volume to a different UID/GID: something like sudo chown -R 991:991 public/system (the default UID/GID of the new in-Docker user is 991/991)" https://github.com/tootsuite/mastodon/releases/tag/v1.4.1

ほうほう…