head over to
Koiphen.com a good amount of "pond liner" installs for koi to goldfish, including many folks with dogs, turtles and other aquatic life.
my initial re-action = you and the pups are going to be eating water a lot.
as soon as some algea grows on the liner. it will be slippery than snot. and you will need to crawl your way out.
your overall sides are "slowly gradual" to center. resulting in. even more frustration. having steeper sides right off get go (of what ever your soil will support), = water gets deeper quicker. this is a good thing. meaning you can "keep more of your body submerged" in water, before getting to edge. and not falling flat on face trying to climb up a slime floor of algea.
i would also look "TPR" trigental point returns. to make it simple. it means creating a "circular current" kinda like a tornado. as water spins around. this will help overall water quality. other words put a 90 on the water coming into the pond. so water shoots down the sides of the pond. vs pointing straight into the pond.
you might consider pulling the liner back now. and digging a trench, and install a bottom drain ""DIY 4" toliet drain"" it will make life easier with getting the muck out of the pond. and it will collect muck, twigs, leaves, and will become a sewer. dogs go in, they come out smelling like sewer. once the muck hits the air they will be nasty. as long as the muck is under water you will not smell it, it is once the muck comes out of the water and into the air.
if this is more dog training. you might consider tossing down 1 layer just for holding water, and then tossing another layer on top that is more for "nails" to puncture vs the 1st layer that is meant to hold water.
those dog nails can be rather sharp to point knife sharp, once the dogs start using them to pierce and claw there way out.
i would not really advise putting rock in the bottom. or less that is your plan to deal with dog nails. rock in bottom of pond = nasty smelling mess to clean. if you do plan to go with rocks... look at "aquascape" i think that is correct company. at some of there systems, and also post something on koiphen, they may help direct you a bit better.