Hi,
I’m not a docker user but I suppose you should modify the file /usr/local/tomcat/webapps/ROOT/WEB-INF/xwiki.cfg (inside your container) and change the line:
# xwiki.superadminpassword=system
by
xwiki.superadminpassword=myPassword
(don’t forget to uncomment the line).
I hope it helps,
Guillaume
EDIT: you need to restart XWiki after the operation to enable superadmin.