luci-app-dockerman: Update to play nice with recent changes

https://github.com/openwrt/luci/commit/180d39dc

 broke dockerman's appearance in the menus

Make a small change to dockerman's index entry to make it play nice with the new architecture, firstchild() was obtained using DAWN's controller/dawn.lua as an example

Signed-off-by: Andrew Dodd <atd7@cornell.edu>