The output from which doesn’t look like an issue, that’s just the launcher looking for the right name to use for docker, but lines 2, 3, and 4 of that look like trouble.
flag provided but not defined: --format
See 'docker info --help'.
stat: cannot read file system information for ‘/var/lib/docker/aufs’: No such file or directory
The version of docker found doesn’t seem to be the one launcher expects. AND you don’t seem to have aufs, which is needed for a working Discourse install.