The image tools are usually provided by the Docker image. Take a look at the Dockerfile and the install scripts install-gifsicle, install-pngcrush and install-pngquant. I’m not sure if all of those are still used by Discourse.
I installed the following packages on my Ubuntu development VM in order to get rid of the warnings.
sudo apt install advancecomp jhead jpegoptim libjpeg-turbo-progs optipng npm nodejs-legacy
sudo npm install svgo -g
Some of them are already installed by the script mentioned in the Beginners Guide to Install Discourse on Ubuntu for Development.