Please advise if/how you can install the plank dock in Antix-15. Thanks.

package for plank is absent from the debian"current stable" (jessie) repository.
You would need to add"testing" (stretch) repository to your sources list.
After doing so, plank should be one-click installable via synaptic package manager, or via commandline:
sudo apt update && apt install plank

IIRC, plank expects to operate in a"compositing" desktop environment (as does docky)

========= SCRAPER REMOVED AN EMBEDDED LINK HERE ===========
url was:"https://en.wikipedia.org/wiki/Comparison_of_X_window_managers"
linktext was:"https://en.wikipedia.org/wiki/Compariso ... w_managers"
====================================

but iceWM/JWM/fluxbox are stacking, not compositing, window managers...
so plank probably won't launch, or won't behave as expected, until you also install"compton" to provide compositing functionality.
You'll need to start compton (manually, or add to autostarts) before starting the plank program.

Thanks for info.