Except for two things that aren't obvious:
a) You need to figure to create the default ticket structures, as described here (github closed issue search is your friend): https://github.com/docker-library/redmine/issues/111#issuecomment-379728491
b) To reduce fragmentation, btrfs users might want to re-create /usr/src/redmine/sqlite/redmine.db to add chattr +C
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